I wasn't able to get the car up to pop the cap and see but I did come across the same info you posted here before jumping into this. I did have a look at the Boost pressure and vacuum levels at the VAC Amp on the left fender. The boost pressure is a smooth incremental transition as is mentioned in Steve's article referenced above. The modulator valve vacuum output has a range of 17"Hg to 5"Hg vac. The 5" reading was at 60mph as I wasn't able to open up to full throttle. I am going to assume that (according to Steve's article) these are good / normal boost press and modulator vac readings and that the vacuum system is operating properly and hopefully get through the remainder of the Steve's check soon. I did run the car with the vacuum disconnected from the mod valve and I now know what "neck-snapping shifts" are.