The RS4 has a special parallel damper suspension that basically stops body roll, figured it was seem more planted than a Merc.
Really never flogged a C63, so nothing to compare it to. USDM's get Pirelli PZeros as stock, but a few guys I know that are up on overseas Audis tell me that you can get much stickier tires on it stock in the EK and EU.
I have some E-Code tires on my civic that are pretty much r-compound competition tires. It's awesome. They wear out in like 10,000 km, but who cares? E-Code tires can be incomprehensibly sticky and stiff. That probably makes a big difference in handling, I know how big of a difference it made on my civic going from DOT UHP tires to E-Code UHP tires.