did a few test and anything with :950 produces asic timeouts
53:2:750 14.4Gh/s
53:2:850 20.25Gh/s HE 1.2%
52:2:850 20.3 HE 1.13%
53:2:750 13.44Gh/s
54:2:850 20.39Gh/s HE3.9%
55:2:850 13.07Gh/s HE35%
50:2:950 asic timeout
49:2:950 asic timeout
40:1:850 19.5Gh/s 1.5% 30°C
50:2:950 19.5Gh/s 1.5% 34°C
52:2:950 asic timeout
i also tried your best result
54:2:850 20.0Gh/s 4% 30°C
it would seem that my miner is hardwired to work best at 19.5Gh/s
At 53:2:950 I rarely get some asic timeout messages that eventually disappear. It runs at about 22.5 GH/sec (reported temp 29-30C). It has been running like this for long periods of time (cgminer 3.10 on Win7/64).