Depends on the brand, some 275 BFGs are damn near as wide as a 315 Nitto...
Also wider does not = better all the time. If you are running some crazy wide tire on a too small rim like the dumbasses who run 275s on stock 8" wide rims then your performance will suffer. Go with a tire that is flush or slightly lips over the rim and run with it. A 275/40/18 BFG Comp TA is a REAL good tire for a 18x10 rim. Fits perfect, looks great and is damn near as wide as a 300+ on most brands. You can look at actual specs on tirerack, look at section width to see how actually wide the sidewall is because, as I said, brands vary.
I'd be more worried about your suspension if you haven't done any upgrades to it yet. Don't be the toolshed riding around with offroad exhaust and crazy tires on the OE suspension...