Audi doesnt have too much of good sports cars. But there are few exceptions R8 is good, but expensive. And RS5 actually is a balanced car which is rare in Audi line.
So RS5 is not bad. But im sure that new M3 is already level higher than Audi. And Mercedes also doing very well lately, even better than BMW with their new AMG models. New E/CLS AMG S is just better car tha BMW M5/m6 and Audis RS6/RS7.
C63 is all about that engine. I would rather have some base model for cheapest price just for that 6.2 V8, there is no need to spend so much on new C63 when You dont get too much more.
I would still safe money for GT-R or some 911.