Update:
Got it stable at the moment with the flashed BIOS.
Core: 1100
Mem: 1800
Fans: 30% fixed
Temps: 54°C
Speed: 25.5MH/s
Wattage: 120-125watts
Voltage: stock
Will spend the whole day trying to get the hashing speed as fast as possible with lowest wattage. I've seen 30.5 on 75-80watts with these cards by people who know how to do this properly. If I can get 100w at 28.5MH/s I'll be happy!
I'm curious, did you try to OC the Memory just with the Wattman to 2250Mhz (and maybe downclock the Core) and see what hashrate and power consumption it gives? I'm interested in the results because right now I really don't know whether or not to buy the newer RX 5xx cards or get second hand RX 4xx cards.
Nope I didn't. I installed TRIXX software from Sapphire to overclock the memory but as soon as I changed default mem speeds it dropped from 22.5MH/s to 18-20MH/s. When I decreased the speed on the core I only got lower wattage and temps.
I did a BIOS flash with a lot of help from @Apneal and had some small issues (green/red pixels) which is fixed now. I took a RX580 because not a single RX480 was to be found new here. I took the Limited Edition of the Sapphire because it also was the only one in stock.