Like I always say, not all lenses are created equal. There is always a sharper lenses of the same design, if you look for it. I have a very sharp 24-70 and it could resolve 22 mp. As for the 16-bite color, most subject you shoot, you cannot tell the difference unless you put them side by side (with the help of PhotoShop, you will never could tell the difference). Both camera have its use, but for landscape work, I go with the MF back.
I also have a P45 on a Mamiya, which is used on special projects. I like using the older manual lenses, because they are cheaper and they are manual aperture but I really don't need the auto lenses. I am from the old school, AF is no big deal. One thing that dazzail did not mention is that you also have to get the Mamiya manual focusing screem, without it, the manual lenses will back focus.
The bottom line is up to you, if you want to stay with the Contax system or switch to the Mamiya system.
If Canon's lenses could fully resolve the 22MP sensor then I would agree to an extent, however they are not even resolving 16MP.
Add to that the 16-bit colour and appearance of the files straight out of the camera and I think the overall improvement would be considerable.