Quote Originally Posted by Ouch!
You can do really poorly, it doesn't matter much. When you get into the line and march, you get an item depending on the television ratings.

Later, when you perform for the president, you get an item depending on his mood. I think.
PERCENT NAME TYPE
1% - 21% = Grenade Item
22% - 39% = Potion x6 Item
39% - 50% = Ether x6 Item
51% - 99% = 5000 gil Currency

(note that this chart isn't 100% exact...)

And for performing Rufus
SCORE NAME TYPE
000 - 050 = Silver Glasses Accessory
060 - 090 = HP Plus Independent Materia
100 - 200 = Force Stealer Weapon (for Cloud)

Found this from here. Click from Mythril Mine/Junon Town and scroll it down a bit and there it is.