I dont think we will go reforge more mastery than its current state. The extra rime proc will work like a "no rune av. + no frost strike" times. Dont think anybody will spam HB twice on a rime proc.
From my point of view , we might end up reforging from crit and mastery to hit , to get white capped (which we will be close with 397 stats) and have that KM procs way more often. Which I'll test on a pre-made character. If it work out , we might see a big difference on dps meters , favoring us
I see frost pulling off 100K+ Obliterates with their main hand.
OT: To be honest, i've wanted to try a Shadowfrost build for a while, but to be honest the new hit/haste 1h swords in the new dungeon are enough to keep me firmly in my "haste stacking" place. Although despite loving haste, i still haven't reforged the mastery on Essence yet :P