So far all we have ever seen in any shows is the first time construction of a lightsaber. Which in no way tells us anything about modding them or fine tuning them. No one has ever defined exactly what modding the crystal means. could be placing the crystal in the optimal position. could be modifying the shape. Since we don't know what it is how do you know it is discipline. And I really have to agree with Otter Jethro. Part of the game balance in this system isJedi need a lot of different skills and other XP sinks. So that while a Jedi can be amazing they are not as focused as other characters which allows other characters to compete. And you never addressed the flaw in your system. It makes fully modding an ilum crystal not really possible as you run in to the difficulty cap.
Besides my flavor reason
the Idea that the Jedi need yet another XP sink seems a bit much.
they already need more base skills then every other career on top of that to be effective they need at least two spec.(lightsaber/job) then there is the amount of XP needed to use the force powers and on top of that to use the force power effectively they need to get at least one or more additional force point from there trees so why would one less XP sink make them overpowered ?
to the last part I addressed that many post ago. as I said then I hear and agreed with what some poster had said on that and was going to change it to can be Modded once per Force rating.