Well - for me, it was the other way round. The way APC is implemented on the older cars, it allows humungous low/midrange torque because it operates close to the detonation threshold there where non-electronically controlled wastegate systems will leave a fair bit of leeway there. The 'octane needs' of a petrol engine seem to be highest at or around max torque.
But at higher rpm levels (from ~3,500 rpm upwards) APC severy tapers off boost. Why? Because it simply isn't very good at detecting high speed knock. Therefore, the APC engineers have chosen to drop the boost ceiling progressively with rpm as not to rely on the knock detection circuits to a large extent.
For a standard or slightly modified car, this strategy allows for good driveability characteristics - there's lots of midrange torque for overtaking, so the car is faster in real-world terms than the hp rating suggest.
From a certain level of tuning, however, this interferes with the nature and limitations of a FWD layout and the classic 900 gearbox in particular. There's only so much torque the gearbox can take, and frankly taking the range of gearings available in c900 gearboxes in mind, you couldn't transfer much -if any- more torque at the wheels to the tarmac in the lower three gears anyways (so much for spending $$$$ on 'bulletproof trannies' then ). When that point is reached, you want to concentrate on extending the torque/power curves towards the high end of the rev range instead of creating ever more wheelspinning, gearbox-breaking low/midrange torque.
That, and in my case APC was never very consistent or reliable, even after swapping out several control units, solenoid valves and knock sensors. I dearly wanted to take out one variable out of the equation and so decided to ditch APC and settle for slightly less boost than APC would allow for using a Forge bleed valve type MBC. This gives me for instance an initial 1.05 bar peak boost, settling at 0.95 bar up to the rev limiter. APC OTOH would max out the boost gauge initially, then drop back to a more sensible level.
Instead of maximising turbo boost, I decided to optimise the mechanical side of things, particularly lightening/balancing rotating parts, and airflow at high rpm levels. This doesn't give me the biggest bang for buck if you consider the engine alone, but it allows me to maximise performance within the constraints of my car as a system .