Hey Joe, welcome to SAU!
Nice choice of car, you should be pretty happy with it overall.
There is a bit of debate as to how much boost the M35s run at standard and with a freer-flowing exhaust, some people HAVE found that they do get around 15psi. Power and torque figures on a chassis dyno will be very different to that given out by manufacturers, as they measure them on an engine dyno out of the car. That's why power figures are always much lower than manufacturer-quoted figures (because a chassis dyno measures power after some of that has been lost through the gearbox, diff and driveshafts, etc.), and torque figures are always higher (because a gearbox multiplies how much torque is sent to the wheels).
As far as getting your car to "slam you back in your seat," don't forget that with you in the car, a full tank of fuel, oil and water in the engine, and everything else, a Stagea will weigh in close to 2 tonnes, so it's far from a race car.
Future mods you could contemplate: A FMIC would be handy (it won't give you more power, but it will give you more potential for performance with cooler intake temps), and with the ECU upgrade, keep an eye on the thread started by Bis-e Bee.
I'm assuming that when your friend smoked you in his XR8 (on a race track/quarter mile, right?) that you didn't stall the Stagea up against the brake before take-off? Also, the XR8 (if it's a BA version) has 260kW and 500Nm (these are manufacturer figures at the flywheel) in a car weighing similar to a Stagea.