This is just a guess. The warp field of the other ship would be inside the shields, so weapons fired at warp wouldn't enter an enemies warp field unless their shields are down. Once their shields were down they'd have to drop out of warp or risk a hull breach from spatial objects striking the hull at warp.
As far as firing at warp, the object being fired should start out at the same speed as the ship doing the firing due to momentum. An increase of force behind the object being fired, whether torpedo or phaser/disruptor, would be sufficiend to get the weapon to the target.
I may be way off with all this, just an idea.