I try to do kara every weekend.
I tend to go with a group of friends, who know what im doing, and actually expect me to fuck it up at some point
The group composition we used last week was: Group 1 pala tank, priest healer, pala healer, shadow priest, mage.
Group 2 : 4 shamans 1 boomkin.

We have done it with just a warrior tank too, in those cases my main OTs for a second or two, the DPS is usaully enough to take a mob down before it reaches us.
If the raid lets you pull, youll do much better, especially on the mobs before attunmen.

The SP in grp 1 increases spelldamage with 5%, the boomkin in my group increases my crit with 5%.
Last run with this setup gave me a DPS of 1350 with my main on Midnight.(other 3 was 1200-1280) With just 808 spelldmg, crit was around 46%.
Only thing thats a real bitch is the trash after curator.... Bloody magic immune fishies!

The real challenge in Karazhan is Shade of Aran, the movement involved...
Prince movement is easy enough. Fire elems on Illhoof makes him trivial.

The raid wont be slowed down except maybe on Shade of Aran and the trash after curator, its faster actually.