Funny thing... In a way, I actually think JG Karthus is somewhat better than Lane Karthus right now.

Considering most of your lane opponents in mid are mostly assassins which if they're good, will pretty much decimate you and while top isn't too bad, its got it's times where you wished you were somewhere else.

In the Jungle however, you've got 2 sections of map to yourself, and if the enemy jungler tries to invade, you've still got the best early game damage available since you start with at least 20-27 AP depending on your runepage, and MR runes are a thing of the past.

My runepage: http://na.leagueoflegends.com/en/featured/preseason-update/8300-8326?build=40100228

My Full Item build: Total AP: 480 (534 if you count Seraph's Mana to AP conversion based on items alone) Note that there's no Jungle item here. That's because Karthus can actually get away with just having to buy and that's it. If you feel like you're gonna get counterjungled, you could maybe get , but just keep in mind that it will set your power spike back.

I will mention, that if you REALLY need the MPen you can replace boots with Void staff, especially now that it goes through 40% MR now (also keep in mind, Karthus's W shreds 15% MR as well)

I dunno, I seem to be having a bit more fun in the Jungle than I currently am in lane. Just a thought for yall :P