According to Mercedes they are. We always complain about not having the same spec car like the ones in Europe. But then again, we don't have any autobahns here.
I've never tried flooring one of the newer ones to check if they are really electronically limited. I did however get my 88 300CE (minor engine mods) up to 145mph on a very small stretch, it was still accelerating. Never tried it with my 95 C280.