My gut is that now that crit is far less "interesting" for demo due to the 30% imp nerf that you could focus more on the spells which means going to the next breakpoint on shadowflame, doom ticks (from a pure damage standpoint), and the 4xT16. Meaning as imps weaken so to does crit's value to the spec...so you could use it as an offspec for aff even with higher haste values.
Does a gutted demo compare to a 3% buffed destro? If they are similar (which my napkin math suggests they would be) you could play demo at 13737 haste > mastery > crit aka an aff offspec without reforge/regemming. Destro stuggles with even the 9778 haste breakpoint making it less attractive as an offspec.