I wish Microsoft had mandated that all Windows Phone 8 devices run at 1280x768. That's the same aspect ratio as 800x480 so it would make scaling up from older devices easy. Instead, we've now got three standards; the two mentioned above and 1280x720.
We're kind of in a weird situation with the current crop of WP8 phones; while our CPUs are more powerful than the one found on the iPhone 5 the GPU has been shown to be slightly inferior to what's found in the iPad 2. While one could compensate for the other that rarely is the case when it comes to processing graphics. Whether that proves to be a problem remains to be seen; there aren't been enough games out there pushing the limits of the hardware.
Honestly, I don't see any reason why we should ever face performance issues, except that many of our games are ports. Outsourced developers are notorious for doing an inferior job and the lack of quality is often quite evident. I wont be surprised if, with the more demanding games, they just cheat their way out of optimizing code by sticking with the lower resolution, taking us back to the initial problem.
I do think the smaller developers are more likely to invest the necessary effort needed in getting things right. It's the big names, like EA and Gameloft, we need to worry about. They're the ones likely to do the bare minimum to get a game on the platform. The game may be crap, but at least they can proclaim they support Windows Phone.