I'm still seeing well over 27 Mhash on ETH on my R9 290s - I suspect it's more the sheer size of the DAG file that is making them slow down from the about 30 they managed 2 years ago, more than TLB thrashing.
No, they're not running at stock clocks - though I did have to run them at stock core clock this summer due to the long-running serious heat wave we had for more like 25 Mhash.
What settings do you use?
They have one of TheStilt bioses on them (I forget which level of undervolt offhand) and I usually run them at 1100/1450 when they don't overheat at that setting.
Sapphire reference models with the blower - which blows an amazing amount of air, I had one die and replaced it with a Delta "screamer" 68cfm regular fan and it was blowing noticeably LESS air.