[Varyon]

Is a Ramscoop (SS1:21) an option? That's TL10 and gives infinite delta-v, provided your drive uses hydrogen for its reaction mass.

Realistically, given you need to speed up the grabbed hydrogen to the velocity of your vessel, there's a practical limit to how fast you can go, but I think it's in excess of 0.04c. Spaceships doesn't comment on this anywhere I'm aware of, however - as far as it's concerned, once you hit minimum velocity for the ramscoop, it turns your drive into a reactionless engine.