Check out the published velocities in the TC Sidelock Manual.
It shows that the .45 (with round balls) & .50 caliber velocity is still increasing at 110 grains, and the .54 is still increasing at 120 grains, with various projectiles fired from TC sidelocks using ffg black powder.
The PDF manual takes ~30 seconds to download:
See pages 75 - 86:
http://www.tcarms.com/assets/manuals/current/Shooting_TC_Side_Lock_Black_Powder_Guns.pdf