In a breech loader the inside of the case is normally a bit larger than the actual bore diameter thus more volume in the initial combustion area. Also the paper of the case & base wad would soak up some Heat, thus lowering pressure a bit. These may more than offset the loss through the vent. I have done a good bit of shooting with cap locks & with a properly constructed nipple never noticed much loss through it.
A former co-worker hunted with a .50 caliber "Hawken Type" caplock. First thing he would do when he had to replace the nipple was to drill it out to the point that when he fired it the hammer would be blown back to Full Cock. His idea was if a follow-up shot was needed he wanted to be able to reload as quickly as possible & not have to fool with re-cocking it.
Only way I know this could be determined for certain would be to run pressure tests on otherwise identical barrels with one set up as a ML'er & the other a breech loader. As I don't have the equipment to do this it is all purely speculation on my part. There are though factors which could point toward either.
