8" is within the range of manufacturer recommended wheel widths for most 245's, so you should be fine. As for how it looks,
this post shows a BRZ with 245/40R17 on 17x8" ET35's - looks pretty good to me!
FWIW,
I currently run 225/45R17's on my 17x8" ET35's, and even with the Pirelli P-Zero All Season's rounder shoulders I wouldn't say it looks "stretched". When I used to run Michelin Pilot Sport AS3's with their squarer shoulders on the same wheels on my previous car, they looked perfect.
That said, when it's time to replace the Pirelli's, I'm going to a 245/40R17 Michelin Pilot Sport All Season.
