I used to shoot Kodachrome at 1/3 underexposure. E.g., K64 at ISO 80. It gave improved saturation and the slight boost in shutter speed was often helpful.
I've always shot E6 at box speed, though, and been happy with the results. These days I shoot mostly Provia and Velvia, probably a 2:1 ratio between the two. I was pleasantly surprised to see the results you've gotten from underexposing your Provia, however. Good detail in the shadow areas still, for the most part. And the softer color saturation I find quite appealing in your first photo.
I'm not sure I understand your exposure rating, however. Did you really underexpose it by 1-1/2 stops? That's an awful lot. I could see maybe 1/2 stop. So what was the ISO rating you used?