Well thanks to Neal's help, I was able to get the car sorted out yesterday afternoon.
Long story short, it turned out to be a combination of a faulty O2 sensor, and a cracked spark plug - we replaced the O2 and all of the plugs. We also dialed in the TPS and IAC sensors, and cleaned the TB and intercooler tubing to get the car running better. There's still a bit of lag with the headers, obviously, but the car now runs great, and better than ever! There are a few little kinks that still need worked out, but we'll get there soon enough...
He even gave me a deal on a new set of Highway Stars interior door trim pieces for my GN.
Just goes to show that there are really great people in the Turbo Buick community, and we all can benefit when we help each other out!
Thanks again, Neal!