Once you are below the DLA of the lower-resolution sensor, you won't get any more "resolution", however the detail resolved should still look better on the higher resolution sensor, as it will be more finely delinted. Beyond the DLA, you experience diminishing returns...that means things could still look better, however as you approach the minimum aperture of the lens, the improvements of the higher resolution sensor over the lower resolution sensor will diminish (not disappear, just diminish).
All of the current APS-C sensors are diffraction-limited by f/8, however diffraction is still so low at that point that it rarely matters unless you really need to resolve something with obscenely fine detail (i.e. you want to resolve the individual barbules on each barb of a birds feathers at a distance of a few feet...you are going to need something closer to a PERFECT f/4 or wider lens along with the highest resolution sensor you can possibly get your hands on...24.1mp would be pretty excellent in that situation.)
I'm still not quite convinced. So I took a image with a 7D and a 50 mm 1.8 II, aperture f/11. I scaled it down to 13.5 Mpixel and up again to 18 Mpixel. Then I compared it to the original image. Even with pixelpeeping I can't recognize that the original has better image quality.