Hey Gman that's pretty much the same sort of power it made before you got it aye, I wonder if the previous owner had something similar happening and that's why it never went further? Might pay to give him a call (if your on speaking terms)
From memory he had said it made 400kw @1.4bar but didnt go any further cause it was a "quick" tune, im guessing they had a similar prob and that's why it was never pushed any further?
Since then it has had a proper rebuild, more fuel system added and the ignition upgrade so you can take that part out of the equation.
T04Z should be up to the task.
Seems flow related so exhaust or manifold???