I have not looked into it but I doubt it is a safe course to assume the market makers are idiots and mispriced the option. Yes you collect a hefty premium but your margin requirement is through the roof and rewards could be staggering only if the index is at 1500 at expiration. Otherwise it is no different than buying the deep ITM call or saving all that margin and buying an ATM call spread with wide strikes. I do not see any advantage to doing such a deep ITM naked put. As expiration approaches, the put price will approach parity or its intrinsic value and lead to loss or offset initial gains. Basically it is a strongly bullish play and you make more money with less risk choosing different strategies.