Hey Tom,
You're spoilt flying a nice modern aircraft; older aircraft there are often few if any manufacturer SOPs so the operator has to write them. So a lot of it is historic. Unfortunately many years of experience has shown that feeding things back to the manufacturer disappears into the abyss - they are not interested in revising things for previous aircraft, they are concentrating on the next one.
Additionally, there is the issue that sometimes the "factory training centre" doesn't actually teach what is in the manufacturer's manual, so if an operator wants to follow the AFM they have to have "company specific" SOPs - go figure!
But the other point of an OPC is nothing to do with SOPs, it is about standards. The LPC is the standard required of a new F/O straight off the type rating course. Any sensible company is looking for a higher standard for their line pilots, especially Captains.
To answer your question of a specific improvement, moving a lot of items to the "after start" area rather than having the PM distracted by reading a great long "Taxi" checklist when he is better employed navigating round an unfamiliar complicated airport is a frequent one.
But to answer the original point, there are regularly moves to try and get common OPCs among operators for exactly this reason, but they usually end up under the "too difficult" file due to the ridiculous amounts of paperwork required to demonstrate it to NAAs. We even struggle to get OPC cross credited within the same company using identical manuals and procedures!
Cheers, BJJ