I have '21 M3P, and have test driven the MSLR and MSP. Maybe the MSLR is slightly slower off the line, but it's for sure better at highway speeds. I'd trade the highway for off the line as the current M3P falls off hard after about 60MPH.
While 3.1 is 3.1 and plenty fast, neither compares to the plaid performance wise. When it comes time to order, I'm going for the MSLR. I don't like the CF interior bits and aside from the speed, the Plaid does not offer anything else value wise for the 15K premium.