Just an observation, at least some of the concerns with scenery not working (objects not visible and such) can be rectified with Arno's great ModelConverterX program (with limitations). One limitation would be that the scenery requires a dll file in order to display certain objects or control portions of the scenery. As we know, if it's a 32bit dll, it will not work. Another limitation when converting scenery objects (for me at least) is if there are animations attached to those objects (opening hangar doors for instance), that requires some extra steps to ensure they still work and I've never quite grasped that concept.
All of my scenery was compiled with the FSX SDK and I have every one of them installed in P3D_V4.1 and every one displays correctly. It's possible that I might pick up some extra framerates if I compiled a version using the 4.1 SDK, but when I'm already getting 35+fps in sceneries that have upwards of 100 different custom objects not including grass. . .why would I? So, if you don't intend to release a scenery package that you re-compile (like MAIW) then learning the basics of MCX is a good way to get some of those sceneries to be usable again. If you want to make it available to others, that's a different story and it would be something that would obviously have to get the approval of those folks at MAIW first.
Just some food for thought.