I've shot cock feather-in since being a kid and taught to do so by my Grandfather. I'm sure he got it from someone like Kelly. I shoot a longbow off the shelf and shooting cock-in does allows me to reduce my nock height and it does give me better clearance.
Just as a side note on safety. Anyone shooting POC shafts should be correctly orienting the grain runout (rift) when installing the nock and fletching. This runout will be oriented for either cock-in or cock-out. You cannot flip the arrows back and forth once oriented as the intent is for the sharp leading edge of a break to go away from your bow hand. If you flip it over upside down, the sharp leading edge of a break will drive through your hand. Breaks are rare but they do happen. I've had two in the last 34years both of which I was saved by proper grain orientation.
Daddy Bear