Going by an Alcubierre warp model, it would make no difference at all. In theoretical physics, warp isn't actually motion in the conventional sense at all -- rather, you're occupying a bubble of spacetime whose topological relationship to the rest of spacetime is being altered by collapsing the space in front of it and creating new space behind it. It was actually summed up pretty well in Futurama
's "Clone of My Own": "The ship doesn't move at all! It stays where it is, and moves the rest of the universe around it!"
So the effective "velocity" of the warp bubble is totally unrelated to the motion of the ship that's inside that bubble. In fact, it probably would be a very bad idea to have a lot of forward momentum when you created the bubble, because you'd just end up flying into the front of the bubble and being crushed by the extreme gravitational stresses of the space warp. Well, maybe that wouldn't happen if the warp-generating machinery was onboard your ship, because the warpfield would presumably maintain a steady distance from the generator. But your ship's kinetic energy would still have no significant effect on the performance of the drive itself, because it would only matter in relation to the "pocket universe" inside the warp bubble, not in relation to the greater universe beyond.
The only advantage of having high forward velocity when you enter warp would be that you'd still have that momentum when you left warp, due to conservation of energy. But that could be a disadvantage, because velocity is a vector quantity, with direction being part of it as well as speed. You might come out of warp travelling in the wrong direction relative to where you want to go, and thus have to waste energy braking and accelerating in the right direction.