Just a quick and simple idea... (well simple in my mind, but I have not idea how hard to code/put in game)
As an engine builder, I understand the vaule of having a good choice and 200% build for a combat engine. Better buff rating on the engine, and more thrust for improved control of the combat situation. Both highly desired.
My suggestion, which isn't a large leap of change would be an additional 200 to max warp or maybe a bit less, for a 200% player made engine vs the store bought. Why you ask? When it comes to selecting engines, two things come to mind. I need one for combat, and one for speed for trade runs or other needs to get from A to B fastest for a mission. The problem here is that anyone can say, why bother getting a player made version of this, since store boughts all have the same warp speed as 200% player made. I undersand there is a Cap on warp speed as well, and as it's set now, I dont think anyone reaches it. From what I've heard, people still fall slightly short, which is good, because that means lvl 9 engines with an extra 200 or so to warp would get folks closer, and still not be game breaking. This would also place more vaule on lvl 9 engines and actualy getting that skill earlier instead of an after thought once skill points are sitting around to spare.
For this can explain why engine building is normaly, by most people, left to very few who take the task to do it. Personaly, I dont mind it to much, because I like doing what most dont. On the other hand, I would like to see a "need" for the engines that typicaly fill the roll of "I need more speed" to be player made to have an advantage over the vendor versions.
This would in effect make engine building more desirable, and give more reason for people to take engine past lvl 6 ... you can read so often how most people suggest that there is little need to take engine skills past 5 or 6(more so because in live the speed cap was 5k), and for some, as low as 4(though this seems a bit silly).
I know there are upteen million ideas and things that need to be fixxed or added, and I hate to toss another one on the plate for you Dev/GM types doing this on your free time, but since you are doing it because you love the game, I just wanted to throw out an idea that might be worth considering and implementing. Thank you for your time you take to read this.
Thorvald