I think some champions deserve to be 6300 if you consider what their abilities are. Ezreal, Karthus, and Twitch's ult can hit every champions in one go from a safe distance too. I think it is a good limiter so people would not over use them.
Anivia: Rebirth, that is automatically deserves 3k ip status. Possible multiple stun with AoE stun, path block, and a nuke.
Blitzcrank: Passive ability can save you from possible death. Rocket Grab, the most annoying ability a player could come across.
Corki: Ehh... Can reveal steath, Gatling Gun can do a great amount of damage if use correctly. His skill shot ult can be spammed to finish off a person.
Ezreal: All skill shots but hits really hard. Has his own flash. Plus his ult flies across the map doing a decent nuke.
Karthus: Global Nuke, what else do i have to say.
Kennen: Can possibly stun more than 1 champ at once with his AoE ult that does a lot of damage too.
Kog'Maw: His ult is ridiculously far but his is still new, so cant really say much.
Malzahar: A fairly long stun with his ult with lots of damage over time spells. Pretty much has his own ignite.