Probably you guys have Hynix vram cards, while those with lower performance, like mine, have Elpida vram. I have 1200/1500 and get something shy of 1.3KH/s per 3x 290 GPU rig.
Old story, already happened with other coins.
Unless you have 290X -cards?
I'm running 1150/1500 and am getting 1805h/s with 3x r9 290 with hynix.