Cam shaft or springs.
The power range of the cam was made for the street reving from 4000-6800. With my old turbos(i still have the dyno graph) made peek power at 6400rpm then started to roll over slowly after 6400. By 6800rpm it only lost 20hp but started to fall a little faster after 6800rpm. Best way to describe the power curve is it looks like a hill on the harizon with a 7% slope.

I built a new kit with twin GT72's. On 20lbs the turbos fully spool by 4500rpm. at that rpm it makes 910rwhp, 5000rpm 1040rwhp 6200 1109rwhp. Makes peak power about the same spot but now it doesnt slowly lose power it nose dives fast. best way to describe this graph is a hill with a cliff on the other side. Could be valve springs not sure yet. THOUGHTS....

Igntion:

I wonder about my ignition. I have a 7al box thats about 9 years old. Could that be why it drops off. Do these fail at high HP if you got a issue? I do know that the rev limiter doesnt work anymore. I cant run my rev limiter in there. Before with a rpm pill in the box it would cause the car to miss way before the rev limit so i pulled the pill. Tonight i tried my 2step at the track and that didnt work so my guess is that doesnt work anymore all together. Is my distributor maybe a problem. Problem being a phazing issue?

I was planing on having a new cam made because i need to pull the car to 7500ish. However i would have expected the car to pull to about 6800 like the old one did. I wouldnt think changing turbos would affect the rpm band of the cam?

 

IMO I would make sure the valve springs are up to the task at the RPM you want to run. They may just be starting to float up high.