Clutch Slipping 2000 GSX 1300R

[Batfink]

At least that can eliminate line issues... only leaves the rod assemblies and return/relief valves OR a warped plate?

There are many ways to check the relief valve for operation instead of a visual check. Remember micro pitting may cause it to stick even though it appears clean and free?
If you have a slightly warped clutch plate the higher operating temps may be cause the plate to amplify it's warping, ending up pushing the clutch pack onto the push rod creating the slip?

Just trying to throw things out there cause I hate the not knowing WHY!!!! as well