- if some of the lenses are hitting the top end of that sensor - would using a Nikon/Sony 36MP test vehicle let us really see where the lens hits its peak? Until Canon brings out their own high MP body, of course...
Putting a finer resolution sensor behind any given lens will get you a little more total image resolution but gets to be dimishing returns pretty quickly.
Like Pi sez, there are other factors too, mostly what contrast ratio is the bottom end of acceptable for these MTF tests. You can resolve more if you accept lower contrast.
On Photozone, you can get a bit of a sense of this if you compare the same lens tested on an 8MP 350D vs the 15MP 50D. If it doesn't max out on the 350d, how high up does it get on the 50d? It'll likely resolve more LP/PH on the 50d vs 350d but the lense may not be able to max out either of those bodies.
On some lenses the result may be very close or at max on the 350d but will it also be max on the 50d? If so, heck of a good lens, if not then we're on that diminishing returns part of the curve where the 15MP body may be capable of more.
AA filter strengths and other processing factors also come into play..
So the only thing these kinds of tests, from DxOmark or photozone etc, are good for, is comparing different lenses on the same body if they're evaluated with the same method. Which, for a given site, is usually consistent enough to be useful for this purpose.
It's a bit of extrapolation to try compare the same lense on different bodies as AA filters and internal camera processing are relatively unknown variables to us.
many grains of salt...