Finally in position to replace my Athena surrounds to match up my M3's and VP100 centre. Would the M2's be good as surrounds?

They are wonderful speakers.

AND, conventional wisdom around these parts is that the excellent QS series speakers are worth the extra money.

I picked up a pair of M2s to use as surround speakers. I ended up with a pair of QS8s. There is no comparison. My unsolicited advice? Save a step. Go straight to the QS surrounds.

The M2's would be as good or better (tonal match) than any other direct radiating speaker depending on the other speaker it is compared to. As the others have mentioned a step up in surround envelopment is to buy the QS series and by most accounts the 8's are worth the extra money.

I'm sure the Qs speakers are great,just can't see spending more than I spent on my mains. Plus They have to be wall mounted and I'm limited to stands

you may want to look around for a pair of Mission 77DS surround speakers. They can be found used for about $100 a pair. They are wonderful and don't give up much to the QS surrounds.