I don't see how pirate could make the supposed 10%/week will the coin sitting in this address.
Even if he's running a ponzi, it would only make sense if the total deposits are a lot bigger than this address balance (which I doubt, but yeah still plausible).

As for the observed transactions, it would also make sense if it was mtgox. Someone bought a lot of coins during the drop, and cashed them out, forcing mtgox to refill the hot wallet.
The best way to check (without MagicalTux confirmation) would be to correlate it to the ask side of their orderbook (supposing it represent a stable % of all btc deposits, which is not perfect, but should still be ok).

Anyway we should soon have the confirmation that it's not pirate stack. Hopefully this shitstorm will be settled in a few weeks