Huh, interesting plugin there. If I find myself using the Arduino libs in larger projects, I'll look into getting VS Pro. I'll keep an eye on it for Express support in the meantime. Thanks for pointing that out.
My goal is to migrate to pure AVR C (at least, when it makes sense), and learn AVR assembly (just for fun, and again, to use when it makes sense). A few of my projects are more burdened by the Arduino core than improved by it, so I'm just trying to broaden my toolbox. The Arduino libs are handy when they're well suited, but why use a crutch when I could do better without?
As I said earlier though, my primary motivation for the move to AVR Studio is to get away from the terrible Arduino IDE. 
It's OK for quick jobs, but with a project of any appreciable size whatsoever, it feels like I'm always fighting with it. I just want to have one foot in each platform until my performance projects can migrate off, and convenience for the others. I do alright using a code editor and pasting into the IDE for quick uploads, but when there are headers and multiple C files, it's not worth the trouble.