Wanted to point out something that I often hear regarding the 1985 870 ECM. Slow just refers to the rate at which it sends diagnostic data. It doesn't refer to the speed at which the ECM itself operates, or that increasing the ECM's processor speed will be better.
By the way, if you are running a stock or aftermarket TPI setup, there really isn't many instances where running MAF would be a HP restriction as opposed to SD. If I remember correctly, the stock MAF with screens flows about 500 CFM.
Assume a best case scenario: 383 TPI engine, at 4800 RPM, operating at 100% volumetric efficiency.... it will consume 498.6 CFM of air. Increasing RPM on a TPI engine from this point on will lower volumetric efficiency, as power drops off.
However, if you had an aftermarket port injection intake like a Holley Stealthram, Miniram, or Superram, then yes, you would be limited at around 500 CFM with the factory MAF sensor, whereas SD would not be.