What average hashrate do you end up with for your RX 6800? Have you tried the April driver + 5.6b?
I have:
Gigabyte OC 16 6800XT - 63.8MH/s using 1314/2140 @ 618mv 134w
PowerColor Fighter 6800 - 61.4MH/s using 1314/2140 @ 618mv 117w
PowerColor Red Dragon 6800 - 62.727MH/s 1314/2140 @ 618mv 104w
using 5.6b April Driver - driver only install
This seems low. Is Fast Timing on and what power limits are you using?
Try my settings:
Cclock: 1200
Memclock: 2150
core voltage: 612
Fast Timing: enabled
Power Limit: Red Dragon is 0, Fighter is +5
And this is what I get using the 21.4.1 driver and 5.6b:
Hashrate: Red Dragon 64.4, Fighter 64.2
Wattage: Red Dragon is 103w, Fighter is 112w.
Temps: Red Dragon is 50C, Fighter is 55C