Not sure where you got those figures from but at least 1 is way off, the quoted 0-62 mph time of the IS-F is 4.8 seconds with most tests placing it below this.
The first performance sedan I drove was a C63 on an AMG day at MB World. I fell inlove with the noise it made, truly a superb car. However, when it came to actually buying and running a car like this it came down to more realistic things.
Reliability, yes MB have improved, but the car I moved out of (SLK 350) had a fair number of issues over the years (and this was one of the most reliable MB cars in recent years) in fact the 6.3 engine is subject to a class action lawsuit for failures http://www.benzboost.com/content.php?1661-Class-action-suit-against-Daimler-AG-Mercedes-due-to-M156-V8-%2863-AMG%29-engine-defects
Servicing costs, not sure if MB was really taking the mick when they decided you pay for a service which doesn't actually include any of the bits that need changing. AMG servicing costs are even higher, prob looking at an extra £300 a year over the ISF
Fuel, ok yep you don't buy a car like this for fuel consumption but at least in the ISF I can get over 30mpg on a long run, and even having fun with it you'll still get high teens.
Dealers, nothing compares to Lexus dealers.
The ISF really is more of a split personality car, you can drive it like a Lexus, enjoying the quiet and luxury while cruising along, or put your foot down in Sport and drive a sports sedan. It manages to fulfill both rolls.
If someone else was paying for it and dealing with it then maybe I would probably have picked the C63 but for me ISF was a more rounded package.