Never any problems with Q stuff in my experience. Comparing their HEPA filters to the genuine one there's sod all difference, bar cost!
From what Stuart has told us/shown us before I'd not be surprised if the materials and manufacturing processes are the same/better than the original Dyson ones.
Also, how can you be sure the Dyson ones don't come from the same factory? Hoover did this in the early 2000's, used pattern suppliers (belts with DBB numbers on) and wrapped in Hoover/Candy branded packaging.