Currently, one of the problems with pirating being hard is that a trade ship can move as quickly as a fighter ship. The number of cu's full should affect the bonus turboing gives you, say, -0.25 per cu.
Centaur turbo: 200
Turbo bonus: 135
Now, lets fill the centaur to the top (48cu):
Turbo bonus: 123
Centaur turbo: 188

Now, an easy to obtain ship, such as a low level vulture, will go slower than the centuar at max speed, but should be able to catch up in the beginning of the chase, so perhaps trade ships should have their armor beefed up a bit to prevent the bad pirates from getting the high end trade ship.