Thanks guys.

I'm currently stable at 53.7 MH/s and 130W.

I used AMDVBFlash 2.9.3 to flash the latest XT BIOS version on TechPowerUp for my non-XT card (ASRock Challenger), ASRock.RX5700XT.8192.19026 from 2019-09-26.
Then I used Red BIOS Editor to reduce the voltage for both GPU and memory to 850 mV, and to copy both 1550 straps to upper memory clocks. The other XT BIOS version for my card has 1500 straps, not 1550, but I couldn't get that BIOS to work so I'm using this one. The 1550 straps didn't bring me any benefit whatsoever.
And then I used OverdriveNTool 0.2.9 to increase the memory clock to 1860, which is the highest stable value. Any higher than that and I start getting AMD Wattman resets to default values.

So, is this it? It seems I've reached the limit with this particular setup, as the only thing that influences the hashrate further is an increase to memory clock, which then gives me higher MH, 54.6 and up, but it crashes within 10 minutes. I could also reduce the voltage further probably, but at this point I'm looking to get a better hashrate as I see others went up to 55, even 58-59.

I'm not unhappy with 53.7 MH/s but there must be ways to go up further.