I drill out the flash holes to 1.2mm in all my Berdan primed 303 cases because of the variance in F/H size from different factories and I wonder if I should do the same (and what drill size?) with my Boxer primed 303 cases.

All my brass is boxer primes and on my serious rifles (varminting/match) I save the new brass for rainy/bored days, then I sort them by weight, then deburr and uniform the flash holes.
I don't know if it helps that much but I figure every percent counts between 600 and 1,000 yards.

I`ve uniformed primer pockets and knocked the burr off the inside edge of the flash hole, but never uniformed the flash hole its self. The two steps never seemed to give me any added accuracy I could see - maybe my rifle nor I are good enough at shooting little groups to take advantage of the improvment?- so I no longer do either.

Unless shooting benchrest/1,000yds, uniforming the flashhole has proven to be a waste of time in everyday hunting rifles for me. I DO uniform the primer pockets and remove the burr on the inside, but that is to ensure better ignition for the cartridges during cold weather. Cold, slow firing pins need the primer seated solidly. Removal of the burr ensures a straight flame path for the primer.