Well, there's no friction in space, and physics are not my strongest point, but doesn't an object in space accelerate continuously and the only limitation is essentially its driving mechanism? No gravity affecting the mass of the object? So, a Star Destroyer, with its huge engines and immensely powerful reactor should be a pretty fast vessel, when outside of combat, at least.

EDIT: If there is something that bothers me about the fact that Ady and ABC's excellent TESB projects both arrive for the 40th anniversary, it's that the rest of the movies will pale even more in comparison... ah! :)