I assumed that the devs would be using nvidia cards
It wasn't a deliberate choice - just happens that my machines have ATI cards in.
Either way, I assume that you're using AMD's catalyst drivers instead of the radeon drivers?
I honestly have no idea - I stuck with whatever Ubuntu picked when I installed it. I'm sure that as we start to look at performance more, this kind of thing will become more critical.
Bear in mind that Lightworks hardly draws any polygons - as few as two per frame - it's fill-rate and transfer speeds that are more important.