Ok, stable now at 57.0 MH/s, 120 W, 56 °C.

I was finally able to change the memory straps, had to use amdvbflash 3.04+ in order to be able to flash the BIOS. Other versions refused to flash the BIOS with changed straps.

I only had to decrease the memory clock a bit, from 1856 to 1852 MHz.