Chu'lak
Lieutenant
^^Trek's used phasers at warp throughout it's 45 year history, and the Vengeance did in this movie too. The myth that they can't be used at warp is a from the old TNG Technical Manual
When the Vengeance's balls-of-death detached, one fired a photon torpedo (with neat new smoke trail) which blew the front of the Enterprise's nacelle to bits.
in Enterprise "Fallen Hero" Reed said they couldn't fire the phase cannons at warp because of particle discharge. he said he was working on the problem, and by the end fo season 1 they had fixed it.