Both DX12 only games run like shit are are piss-poor ports from Xbox One. The only reason they are DX12 only is Microsoft trying to shove Win10 down everybody's throat. There is absolutely no reason for those two games to not support DX11 - ironically, they would probably run better in DX11, as evidenced by benchmarks of Hitman and Rise of the Tomb Raider that I linked to earlier.
I understand that Win10 is the future and there is no escape from it, it's a fact, whether I like it or nor (I don't). I even have Win10 installed because I bought Quantum Break (which is currently unplayable anyway). I don't expect the devs to support Win7 forever, it will have to go, just like WinXP few years back. I am not some kind of crazy Win7 lover.
However, the performance difference in ME beta between Win7 and Win10 is HUGE - on Win10 the framerate is over two times higher! It cannot be explained by DX11.1+ alone.
If it was, it would mean that Win10 offers free 100% performance boost, and it's impossible.
"BF4, DAI, BFH wasn't, why would MEC be any different? "
Because the performance difference in those games is 10-20%, and in MEC its 100%.
Seriously, do you really think it's OK for a game with this level of graphics to be unable to sustain 60fps at 900p on high settings on a freaking 980Ti and i7 6700K?
If it's Win7's fault, then why isn't Division running so poorly on Win7 compared to Win10?