Is it safe to say you may have overdone it with Nunu a little?
Gave it some time to get more data, but we're up to almost 17,000 games analyzed and here are the results
http://champion.gg/statistics/#?roleSort=Jungle&sortBy=general.winPercent&order=descend
So Nunu is the highest win rate jungler at 56% while simultaneously being the 4th lowest playerbase average games on him. Which translates to Nunu having the highest winrate even though players have no experience with him.
Might need to tune back some of those buffs just a little.
EDIT: For those who don't know, playerbase average games is the average numbers of games people have under their belt on the champion. A low playerbase average games means players have a lower number of games on him, I.E. less experience. It's a much more accurate statistic than the same old tired "but low pick rate means high experience!" argument.
ult