Ok, new O2 fixed it! It runs like a totally different car. Part throttle surge is gone regardless of temp, and overall driveability has improved. Tip in is clean, with no stumble or hesitation, and transitioning into boost is much smoother and crisp. I can't believe I waited so long to address this annoyance.

Even better, the ebay vendor cancelled my order since it was out of stock, and I ended up getting the sensor on Amazon for $24 shipped

I used a Denso universal sensor, part number 234-3050. It has a slightly different shoulder built into the sensor tip that I had to turn down a hair with the grinder, but otherwise works perfect.

 

I had a feeling that was it, Its amazing what a new o2 sensor can do especially with gte ecu's they really depend on it to run right.

 

Yeah, I was very surprised that it goes into closed loop so fast. Glad it was a cheap fix at any rate