Only the ATF can answer that, and you better get it in writing.
According to the rule, they don't only consider rear surface area, but LOP, sights/optics, overall length, weight, added accessories that indicate two handed use, and the phase of the moon.
Since one of the compliance options is taking off the brace, you would think a bare buffer tube would pass muster, but if that was my plan, I would probably request a determination letter for my pistol, with the serial number in the letter.
I have always questioned the usefulness of an AR pistol sans some kind of brace. If you don't want to SBR it, why not make it a rifle? How long is your current barrel? Can you pin and weld a muzzle device to get it over 16"?