Diffraction limit is diffraction limit - there is basically insignificant or no improvement in MTF by reducing pixel size below 6 microns when shooting at f/5.6 and beyond. I haven't tried (and don't want to try) 50D, but laws of physics are unavoidable. No point spending $1.5k to attempt to show otherwise.
This is not to say you can't gain sharpness out of 5k+ sensor by using faster and sharper lenses. In fact, when projecting Master Primes, I was nicely suprised they held up 120 lp/mm corner-to-corner open wide at f/1.2 (T1.3).
But then, 120lp/mm is about ~5k anyway, so if not diffraction limit of the lens, it is the limit of other aberrations of the glass that we hit at this resolution.
I'd hate to pull focus with CoC of 4 microns at f/1.2.