If your argument is that a well written .net app can outperform a poorly written C++ one, then that is pretty obvious. It just isn't the typical case you run into though.
If you're saying these applications are as fast as the C++ equivalents or that people are unable to feel the difference, then that's where i disagree.
In another post, you said you thought that managed apps were improving, and I agree. Perhaps in 10 years this argument will seem like the transition from assembly to C, I think managed apps are just going to get more and more popular and will run better. I just don't think we're quite there yet.