I knew there was a reason I asked...
I just picked out my notes and found a test I did with just switching the brass. Now I know a 5 shot string isn't a good sampling, but I wanted to share with you what I found:
Velocity (fps) 18.0 gr, CCI 350 primer and:
1) 1277, 2) 1286, 3) 1271, 4) 1279, 5) 1292, 6) 1279, 7) 1267, 8) 1273, 9) 1265, 10) 1292
1) 1137, 2) 1188, 3) 1222, 4) 1245, 5) 1228
I think the dispersion is worth noting, even with only 5 shots.
I am planning on starting with 18.0gr, and working 5 shot strings with both WLP and CCI primers in .3gr increments to 18.6gr. If I get up early enough to head out to the outdoor range, I'll bring the chrony. Otherwise I'll shoot for accuracy and then chrono whatever gives me my best groups.
In reality, I'm hunting with my 629DX from a tree stand with a red dot mounted on it, so all of this isn't worth a hill of beans, but hey. This IS my therapy after all ...
I will keep you all posted, thanks for your interest.