Edz says, >>> But probably most important is this, you are using only half the power needed to see the diffraction limit.<<<
So, if the idea is to test EPs of a given focal length and not overall telescope performance, I would think using a scope that produces smaller diffraction limited stars in its focal plane would enable you to better discern the resolution performance of the EP than would using a scope where the aberrations and diffraction limit of the OTA produce a large blob and swamp any affect of the EP, no?
The angular size of the Airy patern is inversely proportional to the aperture and proportional to the focal length, so the linear size so is fixed for a given f/#. So, for a given focal length EP, a faster f/# will produce a smaller input to the EP making it easier to discern the EP performance wrt resolution. The doubles don't have to be difficult overall, they just need to be difficult for the EP.