I tried to do a more careful calculation on the difference between 15MP and 18MP, but it is harder than I thought. If my new calculation is right, then it's smaller than 1/7th of a stop difference. Not significant at all.
Not exact at all, but roughly speaking it looks like a factor of two of MP in a fixed sensor size will change the diffraction limit one stop.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.