Another chat with John at WMSS leads me to believe that the replacement APC solenoid may be faulty and causing the problems.
As the wastegate functions for the base boost setting, the APC solenoid seems the most likely solution – I can’t think of anything else, but I don’t know all the intricacies of the turbocharged Saab engine.
I gave Elkparts a call (still there at 17:20 on a Friday) and discussed the symptons with them.
They were very helpful and are going to look into the problem and send me another valve to test.
For now I will leave the car set to base boost (which still seems fairly brisk), and update this site when some progress is made.