Got the Tahoe back. It still has the Reduced Power thing when you start it, but power is not reduced. I'll try backing the speed limiter down to 255 sometime today and see if it's still an issue.

The new gears feel great. It still feels like I'm driving a big box, but the box isn't as big as it was.

And now, for the important bit - the procedure for updating the shifting profile for a gear change works perfectly. This should work for tire size changes too. Aside from the shift speeds being lower, the Tahoe's shifting pattern feels identical to before the gear swap. If you moderator types want to copy it to a fresh thread and make it a sticky, be my guest.