The weight of the new rim/tire combo you're considering will play a big part too. As a bit of a colourful illustration, consider playing frisbee with a manhole cover; not only is it simply heavier, but it also takes much more effort to get it to spin. Obviously, the difference isn't going to be on that scale, but a heavier wheel/tire will affect your handling, and it will affect acceleration also. Now, you can get a light-weight 18" rim that weighs the same as a stock 17" one, but to go big and not add weight will cost a fair bit of money. On the other hand, the jammy lightweight rims have this habit of looking cool, so... Its up to you. You'll get a bigger contact patch, and they do look better in those gaping wheelwells, but I'd consider all the effects before you go putting money down. Good luck. Jim.
Wanna go for a ride in the Frankenstang?