Looks like I unlocked the final superspecial stage, but I quit, because I was too tired to be able to put up with this level of challenge. From how far I got, I would say it's quite similar to Super Mario Galaxy 2's final galaxy in terms of difficulty. Then again, you can play the level with two Tanooki Suits, so it should be notably easier despite there (probably) being no checkpoints at all.
The one thing I don't like about this game are the purple boxes on the map screen. They are random and only replenish once a day or so. I am not sure how many star coins there are in this game and this is kinda annoying, just like being dependent on random chance.
The extra life count is insane in this game. Crazy enough that they added a third digit, but once you hit 1,000 extra lives, you get a crown on the first digit. Now I wonder if the count goes beyond crown99.