The offset will be wrong. The BBS RC's will stick out 5-10mm further on a B5. Typical C4 offset is 35-40mm and B5 offset is 45mm. This will probably space the wheel too far out and give you clearance problems with the fender lip. It will be close though. Only way to tell for sure is to try.
I may be interested in purchasing the RC's if you can't use them. I've been looking for a good deal on a set for my C4 S6. LMK.