I believe 7.5" is about as wide as you can go without poking a bit, +40 at that point would also bring the inside of the tire as close as you would want to your suspension (8" @ +40 I can't say for sure as I'm not crawling under in this weather :P - but worst case you'd need a 5-10mm spacer, which doesn't help the poke issue). In corners even at 7.5" / +40, in turns, or while turning and hitting any kind of bump, might see the tires kiss the front wheel liner.
I forget where I posted the CAD drawings I did of our wheel wells and how tire sizes effect everything... but it's on here somewhere lol
TLR - 8" wide will very likely rub the fender and the wheel liner, at +40 it'd also come really close to a few other things in there.
If you want, grab a tape measure and see how far out from your current rim you want your tires, remember that the fender has a lip and mounting hardware that sticks in to the wheel well quite a bit (I can't measure these for you as they're no longer on my car lol). You'll also want to turn your wheel and stick your head in there, and measure the gap from the back of the rim to the suspension (and I'd suggest doing this to both sides since they're not always square). With those two you can really narrow down your max size (if that's what you're going for). If flushness is your only goal I'd say tend away from really wide rims, much easier to be a hair conservative and fix a "not flush enough" feeling with shims, than to replace everything.
... or end up doing what I did... grabbing monsters and then figuring out how to fix that mistake by chopping off a whole lot of stuff lol