msata is more demanding for power so you will need a good battery (e.g. the 850mAh Miliamp battery). The file transfer (write) speed with the msata is pretty good at 9 MB/sec. But I don't think there is much difference in speed of accessing files from UI. With Rockbox you can play FLAC lossless files also with cue files supported. RB+FLAC should be an upgrade in sound quality from playing mp3 files on apple's firmware, from my experience.
If you can find a thin battery good enough to fit the thin back plate, you can actually convert your ipod 80GB to the slim one using a thin back plate with the msata drive. So it will be lighter, thinner and more portable.