Your current spec looks like about the best you can do under the circumstances. Only thing I'd do differently is move those last 2 points from Killing Machine into Scent of Blood. The proc rate on 2/5 KM probably isn't worth the extra RP regen from 2/3 SoB - need RP for RS, FS, AMS, and IBF.
Also, don't sweat low hit rating too much. Rune Strike can't miss no matter what your hit rating is, and that's where most of your single target threat comes from. As long as you can keep a threat lead on normal boss fights, you shouldn't have to worry too much missing taunts.
For boss fights like Gormok that require constant taunting, carry around a stack of Glyph of Dark Command in your bags just for those fights, and reglyph for them. Replace the Howling Blast glyph since you'll be using Oblit instead of HB on that one. Carry HB glyphs too for when you move on to the next boss.