mem P3 will always be set to P4 core in PPT. You can override this in ODNT w/ a profile later *if necessary*, though it shouldn't really ever be necessary. 'Mem voltage' in PPT and profiling tools (ODNT, wattman, etc) is simply a floor for core (and soc) voltage. If it's set below your core voltage, it's pretty much inconsequential (though I'm not certain if it also sets a floor for AVFS feature, so a reasonable number may still be desireable.) On the other hand, if your Mem P3 voltage setting is above your core P7, then you will run core at that voltage instead. Your actual mem voltage is fixed in the BIOS.
regarding stability - are you trying to get to effective 1450 @ 850, or your setting is 1450? If it's the former, that's likely just not possible. If it's the latter, i wouldn't focus so much on the setting, but rather look at the effective. I have some cards that drop 100Mhz under load (and therefore need to be set ~1500,) while others only drop only 25Mhz (and can be set closer to your 1430 number.)
so if i understand you...after i have to set P7 Core to 860 (everything lower will result in instant crash or core speed 850MHz) i can also set Mem P3 to 860 cause it will not affect the power consumption?
and i tried P7 1400 with 830,840,850mv but nothing to get stable - after 860mv i can push P7 until 1460-1480 (effective 1375-1395 depends on card)
it´s most important for me to reach stable hs around 1900-1950 with lowest possible power consumption. with getting back to vega 56 bios i save 10W/card - thats good..but the cards need 160-170W - to much
Yes, you can set mem-p3 to anything <= core-p7 and it will not affect power consumption. Or you can just leave it set to whatever you have in the PPT, as you will always be using your core-p7 mv while mining.
As for your stability issues - i would guess you're running up against SOC/controller power needs, rather than core. A 5-10mv change in voltage probably wouldn't give you an extra 80+Mhz headroom for core, so it's more likely the 860mv is required for the 1107Mhz SOC (56 still runs 1107 for the top state despite the much lower mem clock iirc.) If you really want lower power, you can try playing some tricks - like locking out mem-p3 and using the mem-p2 as your top state, which seems to use a tiered SOC clock, allowing something closer to the necessary <=950Mhz, which should further reduce power needs. I've only taken a cursory look at this functionality tho, so can't say for sure whether it will work.