I have 15x6 wheels front and back with 195/50 tires on them. No rubbing problems, fuel mileage was not affected, ride is less harsh, no real noticable difference in handling.
An OEM front wheel/tire is stored behind the passenger seat to act as a spare.
Why did I do this? - to be able to rotate the wheels, to have a usable spare, to improve the ride quality, to be able to buy a more easily and less expensive tire when replacement is required, to reduce the tendency to understeer.
This tire size is as close to the original tire diameter as you can get.
Holliwud - you certainly can put the OEM wheel/tire on the front. It will not rub and will work out just fine.
The skinny front tires were spec'd to make the car understeer. MB was concerned that with this rear engine, rear wheel drive, short wheelbased car, that snap oversteer under emergency conditions or on slippery roads perhaps would be a problem for inexperienced drivers. With the rear negative camber and Electronic Stability Control oversteer is almost impossible. There are many people who have symmetrical sized tires front and rear.