Hi
You can go for custom development also if you feel that's going to fulfill your requirement and of course if user friendly. Adding KF to PA should not impact performance (assuming you are not reading all KFs in one planning book itself making drill-down/up time consuming and not user friendly).
The way you have described in your previous post is basically taken care by CTM for planning. As suggested in my previous post you need to figure out generic way of defining these parameters then making it more complex.
I would suggest you to first check with ABAPer the feasibility and develop a pilot solution on the basis of less complex environment, thereafter you can check in detail how this could be made user friendly.
Regards
Rahul