I agree, it won’t ruin the beer, unless too much is added…
The OP is concerned that mashing higher than intended means he’ll have more a “thicker” body. On one of those brewstrong interviews with Charlie Bamforth, they talk about how it really takes a ton of dextrin, all other things being equal, to increase the “body” of the beer. So mashing a bit hotter than you wanted, by itself, will have a small-to-negligible effect on the “thickness” of your beer.
From that, and my experience, I’d say the difference between actual attenuation and potential attenuation has a bigger effect on the perceived body than any of the absolute numbers. I’ve had a Belgian pale ferment down to 1.010 and a Saison ferment to 1.004, with almost the same OG. The pale seemed drier and thinner than the saison.
Doing a forced ferment test gives you a number to “shoot” for. You can still mess it up, like when I overshot my pale, and undershot my saison, but at least you’re not flying blind.
So back to the OP’s concerns, here’s what I’d do. Pull off some wort and do a forced ferment test. See how your wort ferments out as-is. If you’re off your mark, rouse and warm the yeast until you get the FG where you want it to be.
Given that the FG won’t change appreciably, nor will the content of unfermentables contributing to perception of body, still a higher ABV might change your overall perception of the beer. I wouldn’t go over 10% of total fermentables as sugar, although I know some people go up to 20% in some recipes.
Personally I’d just leave it alone, you really don’t know that you’re going to need to fix it. 155F isn’t so terribly high of a mash temp, depending on your yeast and ferm temp regimen you might still wind up with a low FG. I know the Wyeast French saison yeast (3711?) is a great attenuator. Some of the others need really warm ferm temps to keep going properly.
I wouldn’t add any sugar either. Let it finish, then give it a taste. If the FG is too high and you don’t like the body, I would throw some brett in there. It should go nicely with the saison base and dry it out a bit.
The other option is to make another beer to blend with it. I’ve blended a Saison with a strong Tripel to make a really nice beer. Just another option.
You may find it to be just fine as is. 
BTW, what yeast is being used? Not that this will matter a lot, but I know WLP565 can trick you if you are not careful. You’ll think it’s done and then it starts again. Make sure that the fermentation is really finished. I wouldn’t add sugar either at this point.
If it was my beer, I would probably add honey or inverted sugar syrup at high krausen - I have had good experience doing that.
I’m surprised that no one has asked about the original recipe, to establish what might be good/bad…
It’s hard to get a thick body with only a 1.042 OG. It’s probably fine, but the recipe would help us decide.
You could just throw some Wyeast 3711 in (if that’s not your primary strain). That yeast doesn’t seem to care about mash temps.
So sometimes I’m thick. Fred said a pound of sugar in 5 gallons would lower the FG by 0.0027, for some reason I just can’t get my brain around where that comes from…perhaps too much travel.
I’m thinking a pound of sugar is 45 pts (some say more, some say less). 45/5 = 9 pts per gallon, the sugar essentially ferments to completion leaving 1.000 out of the 1.009 we started with. Of course that is a pure sugar/water ferment. Take me the rest of the way to the 0.0027 since I’m not coming up with it…(where’s the head scratching emoticon when needed)
Me, too. Please explain.
I thought maybe they were saying that the hydrometer reading would change because of the increased alcohol. Perhaps it has to do with apparent attenuation instead of actual, calculated attenuation, which accounts for the increase in alcohol.
I thought about that and still couldn’t wrap my head around it. If I threw in a pound of sugar into 5 gallons of a 1.045 beer I take the OG to 1.054. With an 85%AA of the initial wort (1.045), it would have finished at ~1.007, the sugar component should go to completion or 100%AA, so it must be in there somewhere, but the brain ain’t what it once was… 
adding sugar to “lighten body” is to raise the alcohol a touch without added dextrins. if you had a 7% alcohol beer that you wanted to be 7.5% or 8%, adding more malt to get to that extra alcohol would add more body. But, if you add sugar instead, you’ll get the extra alcohol without adding to the mouthfeel.
I certainly hope everyone understands how/why sugar is utilized in brewing…I’m looking for the mathematical proof of Fred’s statement…
Ethanol has a specific gravity of 0.8 or thereabouts. Thats why added sugar will lower the FG very slightly, more of a low gravity solvent in your aqueous solution.
Wine is typically 12-15% ABV, and when it ferments dry it is typically 0.99 FG.
It’s a matter of apparent attenuation.
Let me take a shot at this with the caveat that I need more coffee before I’m fully functioning.
Let’s assume the beginning OG of 1.045 and 85% AA yielding an FG of 1.00675. 1 lb of sugar in 5 gallons would add 1.19% ABV according to Beersmith. So let’s say that adds 7.6 oz of volume (5 gallons x 128 oz x 0.0119) which ferments out completely and produces that volume of pure ethanol at a SG of 0.794.
So we’ve got 640 oz at an FG of 1.00675 and 7.6 oz at an FG of 0.794. That makes the final volume 640 + 7.6 = 647.6 oz
FG = (640 x 1.00675) + (7.6 x 0.794) = 650.3671 / 647.6 = 1.00425
1.00675 - 1.00425 = 0.002502 <---- Drop in FG due to adding 1 lb of sugar.
So it is apparent attenuation then. Here’s something I found on the homebrew wiki that has a calculation that Fred may have used:
Apparent vs. Real Extract
Hydrometers are calibrated for measuring the sugar (extract) content of a water solution. This is true for wort. But when used to measure the extract of beer, which contains ethanol, the reading will be skewed by the lower specific gravity of the ethanol. As a result the hydrometer shows a lower extract content than the actual beer has. This measured extract value is called apparent extract (as opposed to the real extract that is measured when there is no alcohol in the solution) and is commonly used when refering to the extract (or specific gravity) of beer. Like the real extract it can be expressed as weight percent, degree Plato or specific gravity. To determine the real extract one can boil-off the alcohol and replace it with distilled water before using a hydrometer. Or, if the original extract is known, the following formula [Realbeer] can be used to calculate the real extract from the apparent extract:
real extract = 0.1808 * original extract + 0.8192 * apparent extract
[edit] Apparent vs. Real Attenuation
When the apparent extract of the beer is used to calculate its attenuation it is called apparent attenuation. The use of the real extract will give the real attenuation. When brewers speak of just attenuation they are most likely to mean apparent attenuation since it can easily be calculated from the hydrometer readings.
In the case of the OP…he has a beer that starts at 1.042 and ferments down to 1.011 for an AA%=75.
If he adds a pound of sugar (table) at 46ppg/5gal = 9.2 gravity units which will increase his OG to 1.0512. He’ll ferment out all of the sugar (table) and 75% of the sugar from the original wort.
.75 (AA%) * 42 (GU) + 1.00(AA% of sugar)/.79 * 9 (GU) = 42.9 GU
51.2 - 42.9 = 8.3 GU = 1.0083 finishing gravity
So by adding the sugar the AA% increases by 5% and the finishing gravity is reduced by .0025 thus slightly altering the mouthfeel (reducing) of the finished beer.
Bluesman - Your calculation assumes that the sugar that’s added will ferment out to a volume with a SG of 1.000. Really it will ferment out to a volume with the SG of ethanol or 0.794.