multiple / different GPUs in same machine ?

I've currently got 4 R280's (7970's actually) in my server, and am looking at adding 3 more into the same rig - will adding 3 R290's give me the extra cycles per card that I'd expect, or should I stick with the same platform?

I guess what I really mean is - is the load shared equally between cards, or do cards "draw" more work as they finish processing their individual queue?