After unsuccessfully searching for a MBC to borrow, I finally decided to purchase another one to test; I was reasonably confident that this was the problem after everyone's suggestions and my own testing. I picked up a Hallman Pro RX for $110 - best price I could find at a reasonably local place - and swapped it in with immediate results. With the heavy spring installed, the car runs great and the boost control is responsive and finely adjustable. I did not adjust the wastegate arm. The Hallman did have the hole drilled on the WGA nipple whereas the Buschur piece did not. I was thinking of drilling the Buschur piece to see if that would fix it, but I convinced myself that I really wanted a boost controller that did not require tools to adjust; so it goes.

Overall, a successfull fix. Thanks for all the suggestions and followup and thanks for putting up with my inexperience.

 

 

I'm glad your problem is fixed...
I too have the Hallman Pro RX and love it!