Wheel adapters...
well, I haven't tried it but it's my understanding that if you just try to bolt up the C5 wheel the inside surface of the tire/wheel will contact the trailing arm in the rear before the studs even poke thru the holes on the wheel. This is because the bolt surface on the wheel is so far out toward the outside face of the wheel.
The bolt surface on a stock rally is about centered in the wheel.
The C5 wagon wheels that I was going to buy would require a 3" wheel adapter to make the INSIDE plane of the wheel/tire reside in approximately the same plane as the stock rally wheel so that nothing would rub. I might have gotten away with 2.5 or so but the max thickness they would build for me was 2".
Now as you can see from above posts mounting a C5 wheel to a c3 is do-able. I am just letting you know that there could be wheel bearing problems if you do it. If your car rarely sees a lot of miles in a year it may not be an issue for you. My car will get 10K - 14K miles in one year barring serious breakdowns so for me, added wheelbearing wear just to have 17" wheels is not an option.
Another thing to consider is that the rotors and calipers on most cars, C5 included, are positioned close to the inside face of the wheel spokes. On wheel styles that allow a good view of everything behind it, like the wheels you're considering, it's kind of a nice thing to have something cool to look at as a backdrop to the wheel itself, like maybe some drilled and slotted rotors with a zinc coated center along with calipers powdercoated in red or black or whatever. Look at the car with those Z06 wheels. It's kinda' hard to see in that pic but the rotor is WAAAYYY away from the inner face of the wheel spokes. I have never seen a C5 wheel on a C3 in person but it seems that would look kinda' weird to me. No rotor there as a backdrop for the wheel. Just a big empty hole with a GREAT view of a $50 adapter.
I'm not tryin' to discourage you from buying C5 wheels. If you want to put those on your car and they make you happy - go for it. I'm just trying to point out a few issues that you may not have considered before you spend your money.
Sharing ideas... That's what this forum is for...
good luck bud.
lone73