One of the valves wasn't seating. Do a bore leakage test (If only three tuners had done that first )

It came to light after changing to double valve springs. When changing that cylinder obviously you pump air into the chamber to hold the valves up. It was obvious then that air was coming out!!

Definetly the first place to check. Does it rev past it if you do it off boost i.e rev to 4900rpm come off throttle and take it past off boost? Mine wouldn't go above whatver I did.