Back to the dealer with a possibly related problem, got a bit of a brush off; they said that the problem might go away after the transmission had worn in a bit, and at any rate they had not been paid by GM for the repair, it had been flagged "on hold". A bit ominous. Went back today and was told that yes, they had finally been paid, on the 27th.
So yes, dealer did actually come thru on a repair despite me being open about my mods, and yes, GM did actually come thru as well.
On the other hand, still not too sure about the M32 transmission, since the replacement seems to be having troubles of its own.