I got 18" Wheels on my Celica and I have found them to be perfect.
My father has a celica same as mine and hes got 16" wheels and I can say without any hesitation that the performance drop is not noticable when upping the wheel size.
Just make sure you put lower profile tyres on to compensate for the larger diameter.
Much more grip now too, so cornering and braking are improved!
Heres the beast with 18"s
I think I prefer the top wheel though, but they are getting to be common with the smaller cars and the Honda people for some reaason.
