i attempted 5man Onyxia and here is my experience
Group: 1 t4/5 prot warr 4 elemental shamans.

Phase 1, flawless not a problem in the world, everything went to plan needed heals but not too many.
Phase 2, "The oh fuck phase".
The issues i found with phase 2 wasnt the welps so much, it was the fireballs she drops, I was chain healing through them but i was finding mana was a big issue, i wiped at 75%.
Even with FR totem down it was still doing too much damage, to the whole group, without being spread out it was a real problem.

I lated on 8 manned it with a independent tank and healer and lock and everything went much better, however the mana issues where massive too many pots to drink and there was a couple of times where only the warlock was able to DPS because i was just oom.
I was doing it in pvp gear because its the only real set i have at the moment.

Theres my wrapup.