Yeah no idea how you get 290H/s. I am guessing your timings might be modded and its incorrectly displaying the hashrate. I had this happen before with ETH mining.

For Tahiti, the most I can get is 245H/s and that's with a high end 7970 which is maxed out at 1100/1575 with memory straps hacked. The other 280x are all at 230h/s, which is still higher than my RX 470s.