Yes or No and why?

sure why not....

4 door sedan to a sport car...

if you can afford the financial hit.. and makes you happy. Do it regardless of what people say.

I drove the new C4S and loved it... but it seemed more like a toy then a daily driver. I'm sure it could hold up quite well for every day, but I see it more as a weekend car. I like the safety and space offered by an E55. Plus, C4S did not feel faster (although it was very quick), but it definetely made up in handling. Again, I think of it as a nice toy.

SLOWER

Depends where

its slower on the street

I've owned both ( 06 C2, not 4). I'm just gettin old I guess, but I'm not sure that I'd want the Porsche as my daily car, given how many times I'm in and out during the day. But, it is terrific on the weekends, the brakes are fabulous, (especially after the weird feel of the MB brakes), and depending on where you live, there are lots of cool Porsche things to do on the weekends, much more so than MB stuff. Only thing is, if you don't know already, you're probably going to be miffed when you find out what the E55 is worth in this marketplace. Hope this helps.

Ceramic brakes cost a small fortune to replace. If I am not mistaken, the rotors are about $1300 per. So it is concievable that a "simple" brake job could cost 3 grand or more.

Good luck