I've been thinking about this. My in-wall/in-ceiling UIW RSSII's have a very similar configuration to the Gems. They are designed to either mount in-wall on the side or in-ceiling.
When I emailed Def Tech, Re: optimal ceiling placement, they suggested orienting the speakers to fire forward and backwards, as opposed to bouncing off the side walls. This seemed counter-intuitive to me, but has worked out fine in practice.
My point is: I think the bipole speaker configuration produces sound diffuse enough that I can't localize surround material (unless it is deliberately directional) in my system, even when my surrounds are not directly bouncing sound off a wall.
I have noticed, however, that the localizability of my surrounds varies somewhat with the seating position--I avoided aiming the speakers right at the head of the listener.
Bottom line: I suspect the more diffuse bipole nature of the Gems will work in your system as a side surrounds, even though your back wall is relatively far away.
I hope this helps--you might contact Def Tech support if you haven't yet, as they may be able to shed further light on this subject.