Band is likely worn.

Take the servo out...measure the pin length...weld a tip on the pin and then grind it so the pin is .090-.100 longer.
you want band clearance around .060

 

The vane count in the pump is not the reason for falling pressure at higher rpm. It is due to the pump slide spring stiffness. The pump slide (& spring) is intended to reduce pump volume under situations where max volume is not needed. The idea was that it would save a mpg or two and not work the pump so hard...in reality all it did was cause issues and make high rpm volume drop off so much that the pressure would fall and then cause clutch issues

Use the sonnax high rpm pump slide spring...or if you want to be a real shade tree guy, use the stock pump slide spring and slam a 5/16 bolt inside of it with the head sitting against the aluminum pump casting. This will preload the spring making it act stiffer just like the sonnax. It's not a flashy way to do it, but it is effective.

 

"so no matter what the vane number you have high rpm pressure problems?"
Correct. 700R4's were worse. Anything past 5,500 rpm (except hi-rpm Corvettes), line pressure started to drop. People would say that the transmission shifted fine at 5,500 rpm and when they would take it to 6,000 rpm or higher the shifts got softer, or sometimes a 2-3 shift flare would occur. LS transmissions are setup to go to 6,000 rpm maximum. Then after that the pressure starts to slowly drop. I use the TransGo setup on these, for the solution.