That sucks - pardon my French.

On the off chance you don't get any satisfaction from GM, I wonder if you'd consider going so far as to look at spoofing the TCM. Modifyng the TPS and/or speedometer signal comes to mind as a way to force an earlier upshift.

(Side note: some Honda V6 owners - Ridgeline & Odyssey - report being able to trick the TCM into locking the torque converter earlier (a few MPH) than normal by briefly shifting into neutral, then back to drive.)

But let's not get ahead of ourselves. You've obviously done your homework and identified a significant gap between the programming and the hardware that the manufacturer should be made aware of.