Wow, I'm stupid.
In my entire mini-review I failed to mention a little thing called Death Match. Apparently, after beating the game, the main menu gains the aforementioned selection. In it, you can replay all 15 of the Boss Battles and the record your best time for each fight. This includes Shinobu's and Henry's fights as well. I'm gonna be honest though, as awesome as it is and as well as it adds to the replay value, I won't be surprised if some people get mixed feelings over it. Here's why: 1. The battles are all much harder than normal, I'm guessing they're "Bitter" by default. 2. There're no health or battery items that I could find in the areas where it may normally be acquired (the exception so far being one of the fights where destructible objects yield mini-health items, and it just so happens to be one of my favorites). 3. For some unusual reason, you can't pause during the fights, meaning the only ways to quit an go back to the menu are to get beaten, or to press the home button and reset. Neither are particularly time consuming I suppose.
Thing is, when you beat the game and when the credits end, it goes right back to the opening of the game, so I suppose it takes turning the game off and turning it back on to notice it.