There's no great secrets to tuning this engine, its the same with any engine. On the atmos, they fit high lift cams, free flowing exhausts, bigger throttle bodies etc (like our Richard!) and they have got up to around 240bhp, the superproduction running Lucas fuel injection on throttle bodies ran upto 300bhp as Steve said, though at pretty high compression.
It all about getting the air in and out fast and easily enough. The problem with the turbo model, is even though you are ramming the air in, the cams are low lift, and the exhaust manifolds are restrictive, simple as that.
Gas speed basically, a standard GTA at a bar will make around 240lb ft, the one with cams made 330lb, because the gas speed is slow enough at around 3000rpm, that the manifolds and turbo dont create a restriction, and the cams are allowing in over 25% more air. Its only at the revs rise, the torque drops and you get stuck around that magic 300bhp figure because of the increase in back pressure. If you look at the plot, peak power is at 4900, lower than standard, and it drops back to around 270bhp at 5750, but there is still usable power after the factory rev limiter position due to the modified ecu.
I think ,but was looking around £500 for the OEM style box but less retrictive .
CLUB TESTBED 
