Aberrations producing a P-V Wavefront Error greater than l/4 will degrade the lens or mirror's resolution. Aberrations producing P-V Wavefront Errors of l/4 or less will allow the mirror to operate at its maximum resolution.

Diffraction Limited Optics defines the maximum acceptable size of aberrations as having a Peak-Valley Wavefront Error of l/4 when l = 550 nanometers,- so that Rayleigh's criterion for maximum resolution can be met.

However, it is possible to grind a mirror with P-V Wavefront Errors smaller than l/4. The mirror I ground and figured was tested at UBC to have a P-V Wavefront Error of l/24. The aberrations in my mirror were six times smaller than the acceptable limit defined by Diffraction Limited Optics. Not bad, especially considering I never saw anyone grind or configure a mirror before I started to work on that mirror.

Does it make a difference to have smaller aberrations than defined by Diffraction Limited Optics?  Yes.