I'll be trying the M5 as a centre very soon. In my next iteration of my system, an M5 centre would fit better. It remains to be heard if it will sound better in my space.

I think the M5s would work well for you as rears. Do you really need 2? My Onk permits a single rear. How far from the MLP are your M5 sides BTW?

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated