You should be able to set thread concurrency a lot higher than that.

I have a 5x XFX r9 270 rig running Windows 8.1. At intensity 19 with GPU engine at 1040, memory clock at 1400, along with thread concurrency set at 21568 I get a stable 460kh/s per card.

I did not install any custom drivers. When I installed the OS, I had all 5 cards plugged in, and I let windows auto detect and install the drivers.