Some have done this in a vertical position with fabulous results. In fact there are a few people that have done 5.1 with all m80's.

Laying them down would screw up the mids/highs. The left/right axis design would then change to being up/down which would be going against design.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85