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So, with the information in the threads here's how I would settle the question of whether to use the gauge or TPMS to set the tires to spec pressure.
I say one should not rely on the TPMS system for inflating to specification, and one should instead use a tire pressure gauge.
The first reason is that TPMS was not built to be accurate, but to detect dangerous low pressure situations and warn the user. A false warning is better than no warning, and it's better for this kind of system to err on the side of under-pressure. So far, in all of the comments I read, the system was under-reading pressure, which is what we would expect from a safety (and legal) perspective.
On the other hand, I've only observed reproducibility in tire pressure gauges, not in TPMS systems. The measurements from a my gauge are the same as the ones in the air pump up to 0.5 PSI. At least one person in the threads reported the same about their gauges.
So, my operational takeaway for this question is that one should get a gauge, cross-check with other gauges to ensure reproducibility, and use the gauge to set the tires' pressure to specification. After setting the tires to spec, perform a TPMS reset, and count on TPMS to warn you if your tires are dangerously low, but nothing else.
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