CS3 - it is indeed a beast. but it's hard to tame. the M3 offers better handling, traction, and just over all more balanced. M3 line was created to race, CS3 is made to compete with the M3.
RS4 - nice car, but it just lacks that umph. the all wheel drive is cool, but the car just comes off as a bit on the soft side.
IS-F is pretty nuts, and a nice car, but the handling just kinda brings it down a notch compared to the M3.
as for the sedan vs coupe argument. I like the coupe a lot, and realize that it's the base. but im thinking down the line here, i know those 2 extra doors are gonna be worth it in the long term. not to mention, the coupe literally has 2 seats in back.