Apple are putting the P7s on display in their stores now, so it's definitely worth giving them a try. I've just done a quick test with my Moto G and it's driving them fine. They're still built for mobile listening so I doubt that iPhones and the like will have trouble driving them.
They're definitely in a different league and as someone who's already spent a stupid amount of money on headphones they were a no-brainer for me. However it's a subjective thing, and you got a great deal as far as P5 prices go, so you may not think the additional £200 is justified or even just prefer the P5's relaxed sound (in which case I'd say try the Momentums). I'd say give them a go, because in my book they're phenomenal.
Yep, this I intend to do. As you say it's all subjective. Just been listening to my P5's and can't imagine anything better so bring it on! I've got a few weeks to compare but you tend to lean towards what you're used to so will try and be objective.