I have never found a significant difference in performance between cap ignition and 209 ignition using BP and BP subs up to BH-209. With BH you do need to have a very hot ignition source - shotgun primer.

The big difference between the to is cleanliness of the breech area. And the fact that 209 is more trusted because of the sealing of the primer and the fact that a 209 can often over come outside really cold temperatures and even ignite a somewhat damp powder load.

For ML season all I shoot are #11 Mag caps or RWS Dynamit Noble 1075+ caps. I can seal them much tighter on the nipple post than I can Musket caps. And the two caps mentioned above produce the same heat as does a Musket cap, but the Musket cap will burn longer.