In my own tests with cleaned and uncleaned primer pockets in loads used for 1000 yard matches, cleaning them makes test groups change about 1/10th of an inch at that range. Sometimes bigger, other times smaller. Too darned hard to measure with any decent level of credibility. So I quit cleaning fired case primer pockets.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master