How about making the speed of the little debris chunks vary according to the damage per second of the weapon that destroyed the ship? For normal weapons, the game would compute this by dividing the damage per second value of the weapon by the fire wait and then by the hitpoints of the target. Then multiply by some constant to get the correct explosion speed. For beams, multiply the damage by 6, divide by the hitpoints, and multiply by the constant.
It's a realism thing--the effects of blowing something up with a stick of dynamite are markedly different from those of blowing it up with a whole truckload of dynamite.
