At what time does he ask that question Instrument? I am trying to find it? When I watched RTV I thought they were saying you get up to your ships max speed then turn off the afterburn for a much less and reduced fuel cost as you will keep moving at that speed without having to keep after burning. But with the question you mentioned that would sound like there is no speed cap, just limited by fuel to get up to a certain speed if you are after burning. It would probably take an Idris a while as you mentioned to get up to speed that can outrun a smaller ship though. The downside is yah sure you could technically after burn up to a high speed but if you have no fuel left to jump to a station to refuel or if you're out in unknown space well you are stranded essentially.