To be specific, obtaining an 'A' rank in the Cholesteroad's Yoo Who Cannon challenge nets you a pair of Bros. Socks, which doubles the wearer's defense if he's carrying his KO'd brother. In other words, don't bother. Besides, all you get for acing every challenge is the Master Wear, a slightly inferior, physically-oriented alternative to the A-OK Wear.
Say, I've heard that the game's ending differs slightly depending on your level at the time of completion. Has anyone else caught wind of this?