I think the RNS510 only supports up to Class 4 so anything faster won't be beneficial (though I use Class 10 because it's fast to copy from computer to memory card). The more files you have, the longer it takes to load and longer it takes to navigate. When I say longer, it might take a fraction of a second, but certainly doesn't leave me hanging.
I *think* an m3u playlist would play the files from where they were located, provided you don't then start moving them arond. So you should have all the folders as located, and then a folder solely of m3u files that would play the songs in the order you wanted. However, it's not something I've tried on the RNS510.