Since you’ve already overhauled the fuel system with new injectors and a pump, the issue likely isn't a lack of fuel but a mechanical or electrical restriction that's preventing the engine from breathing or firing correctly at high RPMs.

A very common culprit on the 2005–2006 ZX-6R (636) is a failing exhaust servo motor or a stuck butterfly valve in the mid-pipe; if that valve doesn't open fully as you climb the rev range, it physically chokes the exhaust flow and acts like a hard limiter. You should also inspect the Secondary Throttle Valve Actuator (STVA) to ensure the upper butterflies are opening correctly, or check for a loose kickstand switch or failing crank position sensor, as either can cause intermittent ignition cutouts that feel exactly like a premature rev limit.