I think survivability will be an issue with no healer when you reach BC...
Siphon life + life tap aren't really a replacement for a direct healing, especially as the higher level elites hit hard! Even if you go demo, with 5 felguards tanking, there will be times when health funnel just won't cut it for healing a pet.

Personally I would drop one warlock for a priest - you get all the healing you need, a nice stam buff, extra shadow protection & the ability to dispel magic.