Well any concerns I had about the effectivenes of the twin pipes has vanished.
Today I went out to Celicamad's place and run the car on the Dyno plus again, and recorded a clear 35 rwhp gain.
It produced a hold hp of 287 (291 peak) rwhp @ 14psi, dropping to figures in the 270's after a number of runs, idle intake temps were now over 40 deg (let alone on boost), and heat sink set in. These runs were done mid afternoon too, not in the middle of the night with low ambient temps , it was warm in newcastle today.
Also recorded a 5.36 second 0-100 time with two big guys in the car, Toy77(85kg) and me (94kg). So you can make your own speculations about how much faster It would be with only me in the car.
This is nearly 0.5 of a second faster than my previous best( with 2 up).