Now that SuperNOVA revived Extra Stage, you can now do a bit of speed-unlocking.
To get an Extra Stage, make sure your last song is on Heavy in Game Mode and that you beat it with a grade of AA. SuperNOVA has toned down the rediculous "Full Combo needed for AA" scoring system of EXTREME and EXTREME 2 and now gives you a bit of leeway. Of course, you're going to want to have a ton of Marvellous/Perfect steps and as few Greats and zero of the rest.
Another method of gauging yourself is that if you can get 95,000,000 points or higher, that is an automatic AA. That, and you'll hear an obvious warning klaxon at the results screen after your last song.
Here's the songs for each difficulty:
Easy - MAX 300 (Super-Max-Me Mix)
Medium - Healing-D-Vision
Hard - Fascination MAXX
All Music - Fascination ~eternal love mix~
All songs are played with the following modifiers: x1.5 (arrow spacing), Reverse (arrows scroll DOWNwards), Rainbow (gives you a chance to identify the notes).
All of those songs are a pretty accurate representation of the difficulty of the song...compared to the OTHER 10-footers.
Healing-D-Vision is the easiest to beat however.
If you can beat the song (at least with an A; the lifebar probably drops too much to get anything lower), congratulations! You have enabled the song to be listed AND purchased from the shop (all boss songs are 20,000 points).
AND, if you manage to get yet another AA, you get the Encore Extra Stage:
Encore Extra Stage: Chaos
No screwing around! One misstep on the Encore Extra Stage is an instant failure. Chaos is played without ANY modifiers and that's the only thing against you since it's littered with approximiately 42 stops.
Failure to beat any stage merely results with the game taunting you by simply LISTING the song in the Shop, *unavailable* for purchase.