Not that I'm loosing sleep over mpg hit, but it's NOT like several tenths of a 1 mpg. I'm on my first tank with a fresh M1 0w40 and I'm already reaching half tank with like 50 miles less than the case with PUP 5w30 (very thin 5w30).
I suspect I'll end with 22mpg vs 23.5mpg plus.. (I log evey tank and mileage to a T). I'll certainly take it for a smoother idle and more engine protection. PUP also burns in my engine.
I'm basing this on ample discussions about the mileage hit between 20 and 30 wt being almost inconsequential. There are also temperature swings, refueling errors, traffic and a host of other variables that may come into play when going for a clean comparison.
Theoretically, your estimations are very close to my estimations for MPG loss:
1.5 MPG going from a thick 20 your PU P5w30 likely was at end of its life up approximately two grades to a thin 40 would be about "several tenths per grade" right?

There are so many variables, but I hope you get the friendly point I was making. Glad the M1 0w-40 is working out well for you, I just bought some for next OCI myself.