Engine cutting power at 5k rpm
 

When I accelerate hard in 2nd gear, the power seems to rise as expected but seems to rev-limit at around 5.5k rpm. I've connected an ELM327 to the OBDII socket and use the Torque app to give myself some gauges. The boost gauge on that shows the boost plummeting at that point. Is this the cause or the symptom of a fault?
In 3rd gear the power obviously builds more slowly, so it goes past 5.5k rpm, but I've always seem to run out of clear road to really see what happens there (too many Sunday drivers!).

I've got a '05 Blobeye WRX Wagon, running everything standard AFAIK.

What error codes are stored in the ECU ?

There's no error codes, so nothing helpful there.

Best get logging then, see what the Ign/knock etc are doing,