Irridium is the hardest matal with very high melting temp, they dont need to be guaged like ordinary plugs. mine have already done 40,000 K and are still good....
Well I would'nt say that there is any substantial difference in the cars performance but for me
The Idling is smoother
Pick is more vigorous (which means better performance and fuel efficiency)
Engine Sounds great.
One more thing, I only use MOBIL XHP SUPER for my cars, best mineral based oil available in market. It is standard on all Mercedes benz models unless/otherwise specified.