I guess what's bothering me about the bonus movement is that what if you design a ship without its full complement of engines? (Will there be limits on engines or can you just fill three quarters of the hull with them and go at insane speeds like in P&N? I think P&N ships are slower for their engine/tonnage ratios to make up for that...)
Anyway, so you've got an Escort with 1 engine and an Escort with 6 engines.
1 engine / 1 eng/move = 1 move + 5 bonus = 6
5 engines / 1 eng/move = 5 moves + 5 bonus = 10
The ship with 1 engine, while devoting only 1/5 the space to engines, is getting over half the movement of the 5-engine ship! Maybe it's just me, but this seems to go contrary to the principle of QNP (Force = Mass * Velocity)... I don't want to tell you how to make your mod and I won't whine and complain if you don't do it this way, but how about giving each engine an increasing number of standard moves (say 3,4,5,6, to borrow from P&N, or 2,3,5,8, using Fibonacci numbers to get a relatively constant ratio between any two classes of engine, as well as a BIG jump between tech levels) and give an Escort 3 engines per move (or 2 in the Fibonacci example) instead of 1?
Another way to look at it is this:
200kT Frigate, 1 eng/move:
With basic engine, no bonus: 5 moves with 50kT of engines = 5% of space per move
With advanced engine, +5 bonus: 10 moves with 50kT of engines = 2.5% of space per move
OK, you say... Well look at this:
1000kT Dreadnought, 5 eng/move:
With basic engine, no bonus: 1 move with 50kT of engines = 5% of space per move
With advanced engine, +5 bonus: 6 moves with 50kT of engines = 0.83% of space per move
Why should more advanced engines be less efficient with larger ships??? I guess you could come up with some technobabble reason why this would be the case ("the lower-energy warp fields are more easily spread across the volume of a large ship due to their low subspace tension ratio") - and I'd buy it! - but on the face of it it just doesn't seem to make sense.
Just my 0.02 tons of mineral ore...
(Come to think of it, don't most Star Trek ships have only 1-4 engines? At least the Federation ones... can't really tell with some of the alien ships. Maybe you could use the engine mount system that someone (forget who) proposed so that each ship has only a few engines, but scaled appropriately to fit the hull size?)