Just dont see any real point when the HPX records AVCIntra. I guess its an option if youre considering buying a lot of p2 cards as purely a cost exercise. Apple Prores (HQ) and ACIntra perform fairly similarly. And remember if you go the Ki pro mini root you still need to buy CF cards.
Adding the Ki pro mini would also reduce some of the robustness of your set up, not sure what conditions you usually shoot in, but the camera is far more weather resistant than what a ki pro mini is. Added weight, having to cable etc Id say overall there are far more cons than just getting more p2 cards or renting them as needed.
Also agreed. Makes no sense to attach to a 300/370. Although - this device will be a good way to record your live events. $2k cheaper than its big brother. I will be purchasing one to record live events next year.