4th gear boost drop

My 1988 FC Infini was imported from Japan 6months ago and since then I have made the following mods:

Hi-Flow S5 turbo
Front mount intercooler w/narrow diameter piping for good response
Fuel reg
Bosch 550hp fuel pump
Microtech LT10S ECU w/IGBT
Blitz Super Sound BOV
4 Bosch Ignition coils
K&N Pod filter (free with cooler - why say no?)
Blitz boost gauge

Buddy Club racing spec dampers

This setup gives 282.4RWHP without a boost controller however i am going to fit one for more mid range.

Peak boost is 9.5psi which tapers off to redline as per standard wastgate actuator.

This is fine in 1st-60km/h, 2nd-105km/h, and 3rd-145km/h
but when i grab 4th, say at 140km/h the boost will not go above 0.4bar(5.5psi i think...) and the car hits an aero wall cos it's making about as much power as an atmo 10A...

I thought that it could be the pod filter location at the front right of the engine bay, that maybe at speed the aero was creating a vacuum and the engine couldn't get enough air but 3rd is ok at the same speed.

It was tuned by the best respected tuner in my city.

Misc: Lambda starts at stoich then richens up gradually to 11-1 at redline.

The car blew the factory power out the window stock - did the claimed engine power at the wheels... I don't know what the go is with Infini's cos i can't find anything about them. Anyone know anything?

Help with my problem would be useful too... :smile:
Thanks