...For a given firearm, powder, and bullet, peak pressure is very highly correlated with muzzle velocity...
"For a given firearm" is where the rub comes. Since no reloaders are using the "given firearm" that the test data was measured from, the correlation provided may not mean much. And since the data produced a non-straight line, I suspect there are variables at work in the "given firearm" that won't correlate well to other firearms.
BTW, what equipment/method was used to measure pressure in data you provided?