Thank you for the info, if speed doesn't fluctuate with power, I think it's SMI software issue and there's nothing dangerous to hardware, but only wattmeter confirmation will close this question, I'll keep tracking it.
But I don't see the fluctuations on Equihash algo. I should test different algos and mining software to localize the issue. Unfortunatelly I've no enough time today. Maybe in few days I will test some and will responce to you.
I beware the problem becaues of I would like to create optimal configuration for the future rig and the main question is: How much power should reserve above average consumption ( I proposed about 20-25% )
a good practice is to have at least 30% reserve to compensate for any possible spikes and because psu's aren't made to work 24/7 at 100% load (except server psu's and not even those like to be 100% all the time), it will also save you money, because a PSU efficiency is higher when it's load is lower (most efficiency numbers are given for a 50% loaded psu, so a plat 94% will be 90% at 80% load and even less above)