Thanks Chey Mgr, I actually had PIP #4720 done about 30 days ago (or alledgedly had it done)
This time the tech gave me a sheet referencing "DCT P2178: Fuel Trim System Rich at Cruise or Accel"
Quote from the invoice: P2178 Found long term fuel trim at -24%, Replaced dirty air filter, reset trim and test drove now at -16% Spec Tolerance is 0% to -20% at idle.
After I took the Cust Service Mgr for a ride an showed him the issue. I turned the car off, then restarted it. Boost returned but only to about 15 PSI and if I went WOT I can cut the boost out again so this is now reproducible. There is no CEL code.
The only thing on this not showroom stock is the GM stage II and new rims (same tires)