Well I can vouch for the M22s as being a great speaker and will match the VP150. I second the notion of QS8s for side surrounds for the reasons stated previously and the rear surrounds are a judgement call, I have heard 4 QS8s in a 7.1 system and it is very enveloping. I have a 6.1 with 3 QS8s and I love it.
I will guess the seating area is not that far away probably 10-12' so the M22s will work great and direct radiators for the back will throw the sound forward as you will be a long way away from the rear surrounds.
Your idea of keeping the same driver sizes is wise as the M3s vs. M2s/M22s/VP150 do sound ever so slightly different. There are those here that have the M3s with the VP series and report it sounds really good as well so it must be a judgement(WAF) call as the M22s are quite a bit taller than the M3s.
