Surely the wastegate has closed-loop electronic control to ensure a constant boost pressure throughout the rev range (assuming full throttle), why would it deliberately reduce boost pressure at the top end? It must be the case that the turbo is flow rate limited and therefore incapable of producing maximum boost pressure at the high flow rates required above a certain engine speed. Or to put it another way, if you plotted the boost pressure (y axis) the turbo could make against engine rpm (x axis) it would drop at higher rpms.
My theory tallies with the fact that mapping a t-jet with the standard vl37 doesn’t produce great results. You can increase the maximum boost pressure the ECU will allow but if the turbo is maxed out your not going to see any power increase (at very high rpms). I bet those that have re-mapped t-jets with a vl37 produce their new peak power at quite low revs, maybe 140bhp@ 5000rpm Vs the standard car’s 120bhp @ 5500rpm; but they probably get good improvements in low end torque.
In a closed loop system a stiffer spring just means that the ECU bleeds less of the wastegate control pressure off to atmosphere thus increasing the pressure on the wastegate to open and giving the same boost pressure as before. I read that the reason wastegate springs are up rated in tuned cars is to prevent the exhaust backpressure inadvertently opening the wastegate without the ECUs say so.
Stiffer spring means more pressure is required to open the wastegate/higher peak pressure can be obtained and as you said less leakage. Also there is no constant boost pressure as the boost pressure depends on the output of turbo at a given time and the volume of gases spooling the intake turbine(through exhaust turbine), but it will remain at its peak for as long as it can.
I know of cases on the Abarth Forum where wastegates have failed and need to be replaced and this is a fault that has electronic implications as it comes up with errors on the EMS/Fault codes. However sometimes it has failed and there are no fault codes.
The Vl37 will have a peak output that it can put out as a volume of air (cm3) in a given time at a peak turbine RPM, at higher revs the consumption of this volume of air will increase due to the increased RPM of the engine and breathing requirements, which is why the boost pressure (PSI) will decrease higher up the rev range past a given point (To my understanding anyway, please feel free to prove me wrong) As the volume of air supplied will stay the same but the demand will increase.
The Vl36 (AGP turbo?) has the same outside appearance but different internal which seem to allow an increase in the peak volume displacement?.
The low RPM torque figures for the base T-Jet are partly to do with the CAM setup and the aggressiveness of it, for example the Bravo 150 T-Jet and AGP both have peak torque from 3000RPM to something like 5750rpm, but the less aggressive cams in the T-Jet Sporting produce max torque from low in the rev range, 1750rpm if memory serves me well. So the increase in low range torque is purely down to the increased boost pressure that the map allows.
The SS with the same cams as the AGP produces more torque over the same rev range (boost pressure is increased from 1.3 Bar to 1.5 Bar I think, through a different turbo and there are different injectors so assuming more fuel as well) this should prove the higher boost pressure creates more torque
Some of the things you're saying are going a little over my head, my only experience comes from modifying my own car, seeing the results with the boost gauge and assembling/disassembling the forge actuator.
The wastegate itself is not controlled by any electronics, it only works by the air inside the actuator, there is nothing electrical to operate the piston or the flap.
It would be interesting to see some before & after Boost Pressure/RPM graphs for T-Jets mapped with the VL37 turbo, I have such a graph for my car but unfortunately that was with the VL36 turbo.
I think that the wastegate is partially electronically operated as the standard T-Jet can overboost in 4th and 5th Gear even with the standard spring.