Rapid burst targets a specific person (i.e. it targets someone to decide which direction the attack will be in) but does have a conal effect (i.e. it'll hit people around that area). I have never seen the cannon change direction after the initial targetting or chase someone around, therefore I think you could run out of it as melee. As ranged, it's probably not worth it - you'd have run a fair way.
What vastly improved our phase 2s was to have the entire raid close enough that all healers could reach and heal all the groups. We stay spread like in p1 (i.e. a third of the ranged and healers in each 'third' of the room) but we move in to stand on the second circular line 'out' from the centre. All healers are assigned to general raid healing. Stepping even a few steps back from that second line means you're not in range across the circle and can cause problems.
Hope that's helpful.