Can anyone comment on how well M3's match up with QS8's?

Moreover, what is the greatest distinction (other than personal preference) between this set-up and pairing the QS8's with the M22's?

The M3's use a 6.5", no? That would be different tonally, but whether or not that would produce an unpleasing effect is dependent on your tastes and ears.

All 3 use the same tweeter, so you should be OK as far as pairing with the QS8. Sonically, the M3 is a bit more laid-back than the M22, which is more accurate. The M3 will be more forgiving of hash source material and maybe seem to have a bit more oomph, but the M22 will give you that more open, airy sound with better imaging.