XP-2 Super at room temperature?

That's fascinating; here's the CineStill times and the Digibase times in one chart:

Take your pick...or anything in-between, LOL!

That sounds like they may be different formulae -- perhaps even different developing agents (though how you'd call anything that wasn't CD-4 only C-41 I don't know). Of course, that could just mean Cinestill used very slightly different data points to fit their curve and didn't test the extremes. Lots of bad science has come out of fitting the curve to noisy data...

They're both based on CD-4. I really couldn't say whether the difference tracks back to actual product differences or methodology.