I have 15x6.5 et40 rota grids with 195/50/15 toyos and 50mm lowered, personally has just the right proportions.
to fit any wheels in the arch without it hitting the arch or chassis, you'll have to consider the alloy width, offset, tyre dimensions & shape (when wheel's at full lock and/or suspension hit's bumpstop) , ride height etc
with this tyres outer dia, mine just barely misses the arch at bump stop and the inside chassis at full lock.
I think 15x8 might be abit too wide for the standard archway n may start causing conflicts without surgery