It's interesting that you mention the low boost thing; as I saw this while at the drags this week.
Normally I can stall the car against the brake, and the turbo will spool up to 15psi at around 3200rpm. It pushes hard against the brakes, churns the rear tyres, then takes off like a cut cat.
At the drags; the car would struggle to get to 2500rpm, and boost gauge was still in vacuum (it normally has around 5-7psi boost at that point). Once I'd done my run; I pulled over on the return road and was able to stall the car up as I can on the street.
As the engine was fully up to temp, I believe it is some kind of gearbox protection when cold; and it either retards the ignition timing or valve timing (or both) to limit boost. Once I was off the brake it would allow full boost. Throughout this I had full opening of the throttle.
It also does another thing, if I give it too much of a hard time when cold; it will limit boost to 7psi (regardless of RPM or throttle application) until I pull over, switch off the ignition & restart the car. Then it will give full boost.
I'm assuming this is an extension of the gearbox protection, as the engine will be at operating temp, but the car may not have been driven very far, thus a cold gearbox.
Is there a gearbox temp sender that could be giving erroneous feedback to the ECU, and limiting boost, retarding ignition timing and/or valve timing and giving you woeful fuel economy & performance?