Each extra inch causes roughly a extra 10 feet per second on the velocity. So your really not going to see much of a difference over a Lee Enfield with service ammo.

Handloads you can't accurately say as it is a handload and will differ person to person. The bullet weight will make a difference, how hot the person chooses to load it (and since it is a P14 you can likely load beyond what is published in reloading manuals as those would be targeting the weaker Lee Enfield actions), etc.