Check the thickness of those new plates. I got a set of them, and they were all .131" thick, which is the worn-out spec for Honda's OEM plates. When they are too thin, the clutch pack is too short. This prevents them from being squished between the pressure plate and basket in enough strength to squeeze the oil out, so they slip.
OEM plates ranges from .144" to .141" new, considered worn out at .131" or less.
The modern sportbikes with 8, 9, and 10-plate clutches use the same ID/OD as these bikes, and I suspect the vendors are getting careless in their 'cross reference' applications. The way I have been telling them apart in a glance: the thin ones have a groove across the base of the metal tab that engages the clutch basket (this is the "plate dog" to the basket). The proper ones have no groove there.
In the early bikes (K0/K1), Honda had an extra steel plate at the base of the stack, used 6 friction plates instead of 7 as the original clutch. In the K2 after 1/72 or so, this changed to a 7-plate friction stack, with the steel one gone and the rear hub surface ground flat to meet the cork one. Today, you could possibly just add a steel plate to the stack (on the bottom) to make up the missing thickness with those plates. It comes out pretty close to 1/2 a plate thickness, overall. Use the Honda springs, though, as they will be compressed further, so HD springs will strain the hub when tightened this way. Also, make sure the lifter does not bind when the clutch is pulled in: the adjustment will be close. It will get better as the plates wear in, though.
The slipping is most likely happening before the oil gets thin enough to let the plates grip with only the light contact they have now.