I have tried four different bullet configurations in one gun, first with traditionally lubed & GC'd for a baseline, then the following... PC before seating GC...seat GC after PC... GC based boolit but only PC'd with no GC applied.
Every PC boolit performed better than the baseline rounds, (2-2.5 groups with baseline and down to 1-1.5 for PC), I could see no appreciable advantage to GC or non GC as long as they were PC,d.
The test gun was a scope mounted 94 win in 30-30 with a 170 gr gas check shanked boolit from a Lyman mold using coww lead (no tin or antimony added) sized to .311. GC's used were traditional Hornady thick copper, not the alum. can ones.