I don't know how much this may help but I checked some 140XTP recently. The load was 1/2 grain under max (18.5 IIRC) usin 296 and CCI mag primers in WW cases. Avg velocity from a 3" 66 was 1175 fps and 1272 from a 4 5/8" Blackhawk.
I think if you want max speed in a snubbie use the powder the manual lists as giving the highest velocity.
Ernie