3% fee as far as I know but I'm pretty sure that is already paid as soon as you get the coins so you will get paid out the full 15BTC. Don't quote me on that last point though.
You're correct sir, fees are deducted as soon as your deepbit account is credited with the previous block's earnings, so what you see for your balance is post-fees. I have mine set to 1BTC instant payout, and I get exactly 1.00 BTC transferred to my wallet.
cent