I have finally identified and fixed the boost leak! After watching a few videos online, I purchased a series of pvc plumbing end caps and epoxied an air hose fitting to one that would fit in place of the intake pipe between the turbo and afm.
A compressor was then connected and the regulator adjusted to provide 15psi. Safety glasses are a must in case anything pops out. The idea is that any leaks will be audibly apparent and pinpointed by feeling for the air escaping or spraying soapy water to make bubbles. Immediately, a loud hissing was heard from near the blow off valve.
Disassembly revealed the connecting pipe was undersized and had been padded with an extra piece of rubber hose. The result was not much of a seal and a leak at modest pressures.
With the bov hose plugged up and the vacuum lines tidied, the car is driving better. It comes on boost slightly earlier, with the top end noticeably improved.
Interestingly, despite feeling faster, the car is now making about 1 psi less boost and the Turbosmart eboost controller in the cabin is now reading vacuum as well as boost. Eliminating wastegate creep with an electronic boost controller and upping the peak boost from ~14.5 psi to 16/17 psi should see a big spike in mid range torque. It might be worth trying to reconnect and troubleshoot the Turbosmart unit now that it is reading properly.