I don't know what this new "high-efficiency" option does in conversion, but I've found that an 80 kbps AAC with the high-efficiency turned on sounds MAGNIFICENT. Plus, I think I've lowered my music space at least 5 times over. I listened to various Radiohead, U2, and Brian Eno tracks with my Dr. Dre headphones, and honestly could only spot very very miniscule differences between the 320kbps and the 80kbps. AAC is awesome. If you're looking to free up a ton of space, this is a sweet idea.