So, new nFactor causes significant dropping of GPU performance. I've got around 38KH/sec from single R9 280x with I=11 (it was 140KH/sec in previous nFactor).
It seems that I need to increase RAM (8GB in the present situation) for setting higher values of Thread Concurrency.
MRC is going to be more a CPU coin than GPU. Miners can decide to don't waste GPU resources for MRC.