I'd still leave the Mangle glyph out of my tanking glyphs with the change. The only discernible threat difference you'll notice is single-target snap-threat. Long-term threat will probably not change drastically, and I don't know many tank suffering from long-term threat on boss fights (especially bears). I either have Glyph of Growl for fights where you can't afford to miss a taunt or Glyph of Maul for extra aoe threat picking up mobs. Glyph of Frenzied Regen/Glyph of SI are just too good to pass up for survivability. Granted, the stacking HP/damage/healing buff has started in ICC, but we bears will benefit from it just as much as DPSers, so don't worry about threat differences due to the buff.
I'm honestly curious how they'll most likely change the feral talent tree in terms of Improved Mangle, since the talent will be outdated and lackluster with the new changes to the Mangle debuff.