Ricky you have to remember that IHI turbos are one of the most reliable at higher pressures. The TD04 seems very well matched for unlocking power higher up in the range, I'm supprised you thought you could vastly improve upon it even with a hybrid?
Post up a graph of your results, this game isn't just about peak power, it's about where that power comes available, where the torque lies, how that power is sustained, the area under the entire curve.
It actually makes my blood BOIL when people throw around a single peak figure. From my perspective anyway, knowing how irrelevant that figure actually is in the real world performance.
Regardless of all of this, how can anyone possibly be dissapointed with 256BHP from a 1.3litre engine?
Perhaps you own the wrong car...