This could so easily be gamed... have a group of "investors" all deposit huge amounts at exactly the same time hack. Wait for exactly the same period of time, say 15 weekly cycles... bank the 150% return on investment, and then all call for withdrawal at the same moment. Some would get paid, some would not, but everyone would be ahead, at least 50%, and if it was organized right, they could pool the payout which would be the original investment at least, so that nobody would be hurt, and all would gain.
Bingo, instant magic bitcoins! Yay!
Lol, since you know this is going to last 15 weeks maybe forget the crew and just get rich yourself?
edit: I'm only trying to patch your system so that you sink alone, arrrr