I had to test this since people where saying different things.
If you add a light engine to a humpback your acceleration drops by alot in the stats, here is a test how its performing.
Left is without engine, Right is with engine.
I used the thunderbolt recharge to see whats going on with and without the light engine.
If you look closely you see that the left side (without engine) gets to the 1 recharge faster at the 2 recharge there the same and at the 3, 4 recharge the right side is faster with a light engine.
So adding a light engine to a humpback is a good idea, unless you can add a other engine.