So I dropped the car off this morning and just picked it back up. Here is the summary on the invoice:
CUST STATES CHECK ENGINE LIGHT COMES ON
P0301, P0302, P0303, P0304, P0451 STORED. FOUND
TSB# 11-116-12R FOR CONCERN. SOFTWARE UPDATE AVAILABLE.
REPROGRAMMED ECM, ROAD TEST, ALL PASSING AT THIS TIME.
At least no new codes came up since the light came back on. When the tech called to tell me that the car was ready, I asked if he had done a compression test, but he said that he didn’t need to. The “road test” was about 10 miles, and I’ve driven it another 7 so far. It feels a lot smoother while idling, but I'm not going to get my hopes up just yet...