I lowered my C5 on the stock bolts. I bought longer ones, but I really would not want to go lower than what the stock ones offer. It was a spur of the moment lowering so I never did take before/after pics.
I'll only add this as I'm sure the results will be the same. The car does pretty good, but does bounce off the bump stops more than it used to. Actually, it never would hit the bump stops so that shows it wasn't using all the suspension. It seemed to firm up the ride, which doesn't make too much sense as the springs are in the same position, just the a-arms moved. Maybe the change where the piston sits in the shock firmed them up???
Anyway, next summer, I'm going to cut a bit of the front bushing to get the front to sit down a bit more, than remove about half the bump stop from the shock to allow a little more travel (~1/2"). From what I've read, this is just enough to keep from bottoming out the suspension. The next step would be to install coil overs. $$$$$
I would say to try lowering on the stock bolts, if it's the same set up as the C5. See where that gets you, then if not enough, just by some Grade 8 bolts in the correct size.