Ethash is even better than cn-trtl wrt being able to drop core clock on vega. Where you may lose 5+% h/r on trtl by going to cclock p0 (852mhz), I saw zero h/r loss on ethash when doing the same, at least w/ stock timings. Should allow you to get your voltage down significantly - closer to 800mv.
why do you need such low gpu clocks if vega is perfectly capable of higher clocks with <800mv voltage?
https://i.imgur.com/8qnLNSB.pngit is vega56 pulse with hynix 20.5kh/s turtle btw
vega56 ref samsung can do the same ...