I like this idea for what it means for immersion: fast hulls that are 50% faster than average hulls become just 25% faster than average hulls when both mount the best engines. I would expect the faster hulls to still be much faster than the others.
However current way is nicely balanced, and that doesn't have to be so easy with percent-based increases. Let's see:
If we make last tier engine to give a +100% speed, we get that a Titanic hull goes from 80 to 160 (as currently), an scattered asteroid hull goes up to 120 (-20 with respect now, that's bad because they are already discarded by experienced players because it's lack of mobility, quite important in multiplayer games), and a solar hull goes to 240 (+40).
In the end, not very cool for gameplay.
And since there is no trouble with engines giving speed to immobile hulls because engines have been excluded from immobile hulls, I would never go that way.
