I had planned on dual P-mags for my IO-320, but after reading of the high failure rate and problems, decided I don't want to risk it.
I'm going to stick with one Slick mag and one Rose ignition for now.
Maybe if they get the bugs worked out and develop a track record of reliability, I may consider replacing the one Mag with a P-mag.
It's a great idea, but it appears the implementation is just not there yet.