The side by side dual centers gave me a 'phasy' center when I tried with my M22's when I listened off axis. I would guess this was due to some comb filtering or some other anomaly of having horizontal drivers close together, it just sounded a little weird.
Personally I think the M2 keeps up with the M80's quite well, just not for extremely loud play back levels. When I tried it out, the M2 was no worse than the VP150 was for anything I watched and with it's better off axis it was the better of the 2 in my one spot that has an issue with the VP150, and I suspect moving up to the 180 wouldn't solve the drop in off axis playback levels you are encountering, but YMMV as the extra width of the 180 does help spreadout the sound better.
Something you could try is angling the 150 slightly toward the off axis position (assuming it is on one side and not both) to see if that helps. This is what I am now using to alleviate my one seat that has the low center level issues, it didn't take much of an adjustment.