I'm posting this for my dad. It's a 2000 WS6 with ~40k miles on it, LS7 clutch, new slave, ram adjustable master cylinder. The clutch disengages a couple inches off the floor and it shifts fine into every gear under normal or even spirited driving, but at WOT and shifts at 6k+ it grinds on the 2-3 shift. The 1-2 and 3-4 are fine. I'm guessing its the synchros but I wanted a 2nd opinion.

 

Check to see if the clutch is dragging. If the clutch is dragging, it could cause it to be hard to shift into gear at higher RPMs. Also, might want to try rebleeding the system.

 

a few things can cause that, the clutch not fully disengaging, which could be a sign of the slave going bad( which new ones can do) or maybe a master going bad. worst case yes the syncro's could be going out 2-3 and that will cause it to grind.