Mine is actual 8.1 Pro. Real deal. So it will be Win10 by end of the month. I haven't had any real issues running JRiver off memory but I would feel better with 4GB of RAM rather than 2GB. Like you, I will be running it in my car eventually. Maybe find something with GSM as well with antennae you can disable if need be. Surface will have something like that soon I believe.
No lag for me. Only have issues if I try to multitask like I do on my PC with a zillion tabs and apps open. That's not a processor issue though.
Schiit stuff might be more tricky. You'll have to check current output on the port. Mine doesn't power the Fulla though that may just be because Win8 needs the registry hack to disable MS power saving BS properly. I'll have to try it.