I have the W-series with QS8's the same as Potatohead. I did simply flat mount mine as per the instructions. However before I went ahead with the purchase I emailed Alan regarding the aiming issue. His response was that he did all of his listening with the W's flat mounted as per standard installation before he deemed them 'similarly good' to the M series. And even though my W150 is a bit low (seat height) I have no issues at all and am thoroughly pleased with the sound and intelligibility of my setup.
All that being said, I would go with M series with FMB's over the W's if it's an option for you. Quite simply any on wall solution will have some sort of compromise compared to their full size counterpart. That just wasn't an option for me.
As for the QS8 mounting - I would go ahead and wall mount them with the included T brackets. As has already been mentioned, they are very forgiving in placement, they do not need to be aimed (especially in a smaller room) and you save some $ on not buying 2 more FMB's.
Let us know what you decide on, but I'm pretty confident you'll be happy with any Axiom setup!