I'm afraid that I've got to agree with the rule on this. Since a gunboat is a light capital ship, I think it makes sense that a fighter should be able to thrust faster. If you allow the gunboat to thrust to catch up, you remove the fighters advantage, and stack the odds more in favor of the gunboat pilot. Unless the gunboat has a fighter escort (which would make sense, since the gunboat is a cap ship), then the gunboat pilot should expect the fighters to be able to get away. But that's just my 2 credits, and I think this has been done to death....