GPz750 won't rev over 8,000 rpm?

Since you have reinstalled the factory air filter box, it might be wise to restore the factory jetting as a starting point.
Based on some internet searching, that would be-
Needle: 4BC6
Main: 110 (112.5 if originally sold in Canada)
Pilot: 37.5

When the PO went to pods and aftermarket exhaust, he or she may also have installed an aftermarket jet kit like those sold by Dynojet. Their needles typically have a very different taper profile from the stock needles and their jet numbering system is quite different from the stock parts. I understand that trying to mix the two will lead to much frustration.

What does it do when you're at the rev limit and you engage the choke?

It dies

Okay. That is a strong clue I think.

My understanding is that this model did have a rev limiter built into the igniter. I don’t recall where I read that and I haven’t tested any of mine to see what the limit is. If that’s the case, I’m thinking perhaps as the igniter aged a component or two (electrolytic capacitors are the usual suspect) changed value and the cutoff threshold drifted downwards.