That mount has been around for quite a while - a lot of guys use(d) it for the P2 series, and I still do. I use it in combination with the Square Jellyfish tablet mount.
I love the stock mount that comes with the P3A/P, but I'm frustrated by the little clamps at the end. They work great with just a tablet but if I put my sunshade on it, the tablet and shade tries to come off the clamp. I may have to go back to the old system.