Have we come to any consensus on which specs work best for each Highmaul encounter? Or which ones that either spec works?
From my experience in pugs/guild groups so far:
Kargath - Holy definitely works well here. Renew spamming on those who get the DoT, and CoH'ing the random shadowstep things. I tried Disc, but PWS spam didn't seem as effective.
Butcher - Again, Holy seemed to do way better here. I tried as Disc, but ran out of mana very quickly. CoW spam doesn't seem to work very well when there are two tanks taking a beating at the same time. Unless you have two Discs spamming two tanks, I guess? I dunno. Both times I killed Butcher, Holy seemed to do way better overall.
Tectus - Disc prevailed here. Mostly CoW'd the tanks and spot-Penance'd anyone who took damage, spammed PWS for the big raid damage thing. Seemed to do much better than Holy.
Brackenspore - Disc again; CoW spamming the tanks and keeping the mushroom around 50-60% with shields, until we need to top it.
Twin Ogron - Disc once more. Holy did okay, but Disc's strong tank healing really helped out, and PWS spam on the fire phases was useful.
That's as far as I've gotten in pugs, so I can't comment on the last two bosses. This is just my experience too, don't take it as absolute fact.