I was really hoping that, during the Enterprise Season 1 Episode: Fallen Hero, when Lt. Reed stated that they couldn't fire the Phase Cannons at Warp, that we were going to get an explanation that would have put this issue to bed once and for all. Unfortunately, all we got was: It would disrupt the warp field (plot device) and that he'd have the problem fixed soon enough (leaving it open so that someone else could write a script, possibly even the next episode, where they worked just fine at warp).
Technically, (And I'm throwing out anything pre-TMP as the concept of Warp changes with the advent of The Motion Picture. TOS and TAS had starships maneuvering and fighting while at warp, almost all the time, anytime they had to fight at sublight, it was stated that they were clearly at a disadvantage because of it) The only way to have combat at Warp is if the ships involved in the fighting share a Warp Field. At which point, since they are in the same area of subspace, they can exchange fire with each other as if they were at sub-light. With all the maneuvering restrictions of being at warp. I can't think of a single instance (Outside of SFC3) where a ship at warp, exchanged fire with a ship at sub-light after the Warp Concept change. Mostly because of the logic of trying to hit something at FTL speeds leaves a firing window of only a few milliseconds.