Hesitant acceleration
 

My 01 WRX bugeye has developed a fault wherein as you accelerate past 3500-4000 revs she hesitates and becomes very sluggish losing all power, she still reaches speed but painfully slowly. Low end power and acceleration are fine but as soon as you would expect the turbo to kick in, it just doesnt. Different people have told me different things such as a faulty neutral switch, others the IAC valve which I have already replaced but for a different idling problem which occoured before this one.
This fault also doesn't throw up any fault codes which I thought was strange.

Any help or suggestions would be very appreciated

have you checked your hoses to actuator / wastegate etc ?

I have not but I will check that out tomorrow

Turbo to intercooler hose came off or split.

are there definitely no codes being stored or has someone just smooshed your engine light on the dash? Could be any number of things but you'll narrow it down much better with a quick laptop diagnostic watching for WGDC/IAM/knock/airflow corrections.