@maaru If u r going to install a single P1222 then there is no need to go for F 15D, u will be better off with the Pioneer one... P1222 is rated at 400 but it is ok to go higher but the max I would recommend is 500 WRMS for continuous use! But if u want to use this sub for a few months u could go till 800 wrms!
One thing you should know is human ears will only notice any change i.e. 3db after we double the power meaning! Human ear will hear a difference at 400 watts and 800 watts and the area between will be unnoticeable... So try to increase the db not power... getting efficient subs will help