Yes - People have been fireforming their brass just like that for eons - neck it up, usually seat a bullet over a decent load and fireform - or use a small amount of fast powder as you've suggested.

If the shoulder is .005" or .010" shorter than the chamber, the brass will tend to stretch at the web. .006 or .007, although within SAAMI and CIP standards, is still .006" or .007" of stretch at the web, stretch than doens't have to happen. If it stretches, it will continue to stretch there, grow in length and have to be trimmed fairly often.

Some people have to trim-to-length, sometimes every shot to 3 shots. If the brass does not stretch at the web to start with, it reduces the stretching later and maintains it's correct length much longer. The brass lasts longer and never separates at the web- as long as it's neck sized only, without moving the shoulder back.
I do mine the way I do, to prevent the initial stretching in the first place.