well... plasma = super hot gas, yes, but _something_ is keeping it all together in a ball, and I always figured on some sort of magnetic bubble around it, which is fairly realistic, but you'd have to actually shoot a device that maintains the containment, which might be fairly heavy. The momentum will be proportional to the velocity, while the _energy_ would be proporitional to the velocity squared.
Now, for a plasma projectile, the damage isn't so much kenetic energy due to velocity, but rather due to the plasma. well, so maybe you'd shoot it slowly, but then it'd be hard to hit something because of the lag time between shooting and it hitting (or missing). So you'd still want to shoot it pretty fast.
Also, remember that we're shooting a LOT of things. The momentum lost could add up. Hell, it could help manuvering, if you include a weapon that fired a large weight at high speed at a right angle to the ship? Get some quick lateral motion, but don't have to turn. Limited ammo though... mmm, the possibilities.