I had a very similar idea
The "cloak efficiency" (which is identical to the FTL efficiency) would determine how close you can get to an enemy before he notices you. This could be counteracted by sensors.
Or
It could be a weird combination of shields and FTL drives. the cloak efficiency determines how fast the module eats through energy, and your crew would have to feed the cloaking modules energy to keep the cloak active.
It kinda makes sense. If you have a huge ship, you need to have to spend more energy to keep the entire thing cloaked.