8 to 9 feet is very sweet. You won't have any issues with multiple drivers from an M2, M3, or M5 fusing at that distance.
The M3 bookshelf sounds very nice. I haven't heard the M3OW but knowing how the M5 and M2OW perform, I'd say the M3 is good too.
There are big differences between the M3 and the M2/M5HP. The M3 lacks a dedicated mid-woofer driver. That affects the timber of the midrange. This sound can be described as less detailed. Admittedly, sometimes I like this sound. But if you want to follow the Dolby recommendations, all speakers need to have the same timber. If you had M50 or M3 fronts, that wouldn't be a problem but with your M80s, the M3s would not be timber-matched. Besides, I think you'd benefit with improved emotional impact with overheads that present more detailed mids.
That then leaves the question of whether M5s or M2s are more appropriate. I still struggle between these two. Without a sub, hands-down it's the M5 although room acoustics can interfere and create bass bloat. But with a sub, it's really too difficult for me to tell which I prefer without A/Bing them side-by-side. For those who don't want to fudge with EQ or placement within the room, I'll always recommend M2s.
The M5 has the same bump the M3 has around 100Hz. That may or may not be a factor given your floor to ceiling distance. Trevor would know better.
