@ Dan Warner
yes, most of the listed powders are not available to the public, unfortunately. I do believe a few guys would consider to larger wildcats, at least if the right powder would be available.
@all
Let me continue my thoughts way too far. (Note, numbers are not real, it's just an example) Let's say a major manufactur makes slower powder. Now, anyone who wants can neck down 20mm to what ever(15,14,13,12mm...) because slower powder is avalible for use. Let's say rifle specs: 52" long barrel, action 2.5" thick, weight 80+Ib. So let me continue the idea. In 5-7 years people want more. Somebody get a great idea. "Hey let's neck down 30mm to 17mm or what ever" ohh wait it would take slower powder, again. Okay don't worry, a major manufactur makes slower powder, again. So specs, 3.5" action, 65" barrel, weight 150+Ib. Now, you need a forklift or crane to transport your rifle. I want to say i see the end of the road. That's the reason, higher operating pressure will be necessary to make the cartridges perform better without going larger and larger in size.
I'm not talking about the 14.9 or any other cartridge specifically. I also know talking about neck down 30mm cartridges is just crazy talk but it was just an example to show how the road ends.