+P or not, even if brass is separated by headstamp, there may be a little variation in case weight and wall thickness from varying lots, but the separated brass batches will be more uniform than leaving everything mixed. When I deeply seat cast wadcutter bullets for use in an S&W 52, I can feel a noticeable difference in case wall thickness if I change brass.

Whether brass separation will be advantageous to a handloader will depend on the accuracy potential of the gun and skill level of the shooter. Most of us won't see a difference using mixed brass, even if the process is second-rate.