All depends on the offset.
No reason why you can't fit 18x8 if they have a suitable offset, the factory rims are 17x7 40P anyway so its only 1" wider!
I've managed to tuck 18x9s in the rear without rolling the guard with a 45P offset
With 18x8 you will need atleast 40P to 45P offset to get them in without scrubbing.
Here is a rough charge by Black200 from AltezzaClubNZ/Australia: