Intensity 20 on these cards gives me way too many rejects, I found 18 is the best setting (< 1% reject).
Stop using intensity... switch to shader multiple threads - xintensity. Intensity is completely antiquated now.
Intensity - raw(?) - xintensity
13 8192 2.90909090909091
14 16384 5.81818181818182
15 32768 11.6363636363636
16 65536 23.2727272727273
17 131072 46.5454545454545
18 262144 93.0909090909091
19 524288 186.181818181818
20 1048576 372.363636363636
21 2097152 744.727272727273
22 4194304 1489.45454545455
So you are suggesting an xintensity which corresponds to between 21 and 22 whereas he's saying a value closer to 18 (93) reduces rejects.
In my own setup I tested xintensity keeping other values constant. Values higher than ~372 (such as 1100 and 702) were causing some throttling issues i'm assuming due to gaps in utilization (temp pretty cool throughout). Values lower than ~372 (such as 176 and 93) reduced the hash rate with no improved thermal performance. I don't have any reject % numbers though and I will test that again later.