Did MG ever proc poisons?
And I realize its poison vs KS and MG on beta, but your suggestion removes MG from that and I haven't seen anything suggesting a shift on the poison side. When I said KS, I also just mean the actual KS hits (specifically off hand) which I believe also benefit from the 50% bonus. The all damage part of spree is fairly neutral outside of those KS hits, but unless I'm wrong and KS hits are somehow exempt from an increase to all damage done then that part is not neutral to slow vs fast.