I have two 5350 batteries thanks to a crappy retailer on NewEgg's Marketplace. I use them all the time and have no issues. I don't really push my batteries to the limit very often, so I don't notice much of a flight time difference.
Yup, that's what I hear. All P4 batteries are interchangeable with all P4 drones (quadcopters). I'm with SweetSVT99 on this one. I haven't gotten my P4P yet and I was thinking that I probably won't be flying any longer than 20-25 minutes on each battery. I thought about getting a quick charger for the car so I can charge one battery while I am using the other.