Went on another blat today with Pokey. Roads were great, traffic wasn't too bad, but the car was not running well. Drivability at very light throttle openings wasn't great and was punctuated by frequent pops from the exhaust. Going up hills that required larger throttle openings seemed better, but part throttle acceleration felt a little blunted. I leaned on it a bit a couple of times to about 5k rpm and that felt closer to normal, but given the issues, I was afraid to really push the engine and experiment further.

Unfortunately I broke the cardinal rule of blatting on a new engine and failed to bring my laptop, so couldn't diagnose any theories on the drive. After arriving home, however, the laptop revealed the problem: either the WBO2 sensor or controller had failed. The ECU was running in closed loop mode, meaning it was continually, and unsuccessfully, trying to lean AFR from the erroneous 9.0-9.2 readings to something more acceptable. Fortunately the closed loop mode prevents massive automatic fueling adjustments to avoid meltdown from just this scenario, so no harm. Flipping the ECU to open loop mode while I sort out the AEM has restored drivability.