I am kicking around the idea of going with 18" wheels all around? Is anyone doing this on a C5? Any pics you can post? Any down side from drivablity stand point?

Depends on what your using the new wheels for.

1. for road racing, 18" all around. Nobody goes larger. Bigger wheels are heavier and the choices for tires are very limited. See picture below. Also racing rims choices are based on weight. Lowest weight rims mean faster lap times.

2. for street bling, 19" rear, 18" front. That provides a better looking ride and reasonable sidewall diameter for ride comfort and handling.

3. for max street bling, 20" rear, 19" front. Tremendous looking, however the thin sidewall dimensions aren't good for handling or comfort. Both bling set ups are heavier because of rim size and the fact that bling rims aren't usually built with weight being a high priority.

18" all around race set up (CCW corsairs, 295 f, 345 rear)
http://farm3.static.flickr.com/2301/1844484677_09b5c7f829_b.jpg
http://farm1.static.flickr.com/251/524310080_a7f30eac46_o.jpg

19" r, 18" front bling set up(Iforged aero's, 275 f, 345 rear)
http://farm2.static.flickr.com/1216/538670807_50234727e4_b.jpg

One of the members in my car club is running 18" C6 Z06 Rims all the way around on his 2000 FRC Corvette.

http://www.shockerracing.com/images/photos/large1/df6495cf11da8e3188aeebcb244caa181.jpg

I am running 18" Z06 wheels all the way around...Look great handles really well.