I`m playing with cooling setup on my case , and at the end i put my MB on the bench to achieve best cooling. So if that is not problem for you don`t lost your time , pull out the MB from your case.I have R9 290 at 1050MHz Core and 1500MHz Mem. +20% pow and GPU go from 90C in case to 75C on ambient temp. about 20-25C.
You can look picture section , and see that any more serious miner`s have their rigs on bench.
EDIT: I have CoolerMaster SilentPro case with 5x12cm fan`s. R.