4x slots hinder performance. I agree with wa1 that you should just consider upgrading to a faster single card. My system is running CF 5850s on 8x/8x and it's fine they get up to 99% usage but at the same time my 3rd card, a dedicated GT240 for PhysX is on a 4x lane and I've never seen it over 83% usage tops. That's while running FluidMark.
You would see a benefit running 16x/4x or 8x/4x crossfire, but you won't be using all of that second card. So yeah, probably better to just buy a better single card... maybe sell your current one if you can (or if it's a suitable Nvidia card, use it as a dedicated PhysX just for fun)
EDIT: Just realised you did indeed say Xfire so I guess it's not an Nvidia card hehe.