I like the core idea, and some of the specifics, but there are some downsides too. To often I've seen players flame, harass, and even report other players for no other reason than they were doing poorly. How many times have you seen "/all report noob [champ] for feeding" when anyone with eyes can see that either a) they're the tank/support, b) the enemy team is specifically coordinated and focused against them (e.g. the carry), or c) there's some other reason it's obvious they're not intentionally feeding. They're clearly not stacking Boots 5 and running into the enemy base, calling out in allchat "come and get me!" "I'm gonna solo baron now" "Who wants to get fed?!"
Point is, the system as described by the OP can be abused, likely would be, and even if not, is needlessly complicated. I suggest instead that players simply be given the ability to single out allies who perform exemplary, the understanding being that the others were either "neutral" or "negative," and those players who are agreed to be exemplary are simply given an IP boost, after the fact. No time limit, no need to 'steal' IP from others (thus creating potential ragers), and no way to gang up on someone who simply had a bad game. Particularly negative players are already penalized IP through temporary and permanent bans from playing (they get zero for days, weeks, or months on end). Better still, this system would encourage exemplary gameplay, not just 'better than average' gameplay; it could also be (more) easily incorporated into the new Honor system by comparing post-game honor ratings. This would even allow enemies to call out good play without giving them the ability to screw someone over.