since I know I may be wrong im interested in a definite answer from someone who actually studied it in physics or something.
but seems to me a throwing weapon the velocity is from the speed it is released from your arm so moving forward would directly modify that and have quite a big relative effect. but the velocity of bow/xbow is from the kinetic energy stored in the bow, your not directly modifying it by moving the position its released from. so if it does have an effect I just can only see it being negliable. but maybe im wrong if its additive but that doesn't make sense to me
It's basic physics.
Aside from increased air friction once the bolt/arrow is in flight, firing a bolt or arrow from a moving system means the speeds are additive, since the arrow / bolt already has speed and kinetic energy from, well, being attached to a moving system; that's beside the potential energy in a drawn bow or crossbow.
Imagine shooting an arrow sideways from a moving car (you can test it with a ball if you'd like, have someone on the ground watch), it does not magically decelerate from the velocity of the car (slowly due to air-friction yes) to 0, it has the velocity of the car in the forward direction and exit velocity of bow/gun/throw/whatever in the sideways direction.