That file is identical to 290X_ELPIDA_MOD_V1.7.rom or even v1.8 on performance edits.
Why you'd get more vs stock 290X is:-
a) at 1630MHz for stock 290X bios the RAM would be using 1750MHz strap timings which are horrendously high latency.
b) the 1610MHz you set in the 390X ROM would be using 1625MHz strap timings, if they were stock timings the gap between the 2 ROM compare would still be there but not as exaggerated, it's further exaggerated due to tighter timings in the 390X mod ROM.
c) there is the 390X memory controller timings difference at play.
v1.8 390 bios with dcuII voltage table VS default 290x bios = 73 points difference.
Here you should see bigger difference IMO, did you reset driver after flashing v1.8 390 bios with dcuII voltage table?