Claymore's CN miner 11.3 gives on my 270X 4Gb 550-551 h/s.
SRB max 546 h/s.
Not bad result, but Claymore's miner without fee and gives a bit higher speed.
SRB can't read hardware sensors from 270X, Claymore's do that...
umhh my 270x is more like 470h/s with claymore, and less with srb.
what settings are you using? oc, intensity etc