Last I checked, the entire OS was GPU accelerated, and most apps that use even slightly up-to-date APIs (so most apps that are actually worth using) most likely do as well. These aren't Android 2.X days anymore. The GPU will do a significant amount of work loading the GUI. Also, an optimized dual core can easily beat an unoptimized quad core, especially in this scenario, since the Moto X technically has more than a dual core. It also has two extra dedicated cores that handle all of the low power processes, taking that off of the main processor's shoulders. On a normal quad core, these processes would probably take up the resources of the one or two cores that are actually in use, with the 3rd and 4th core most likely never kicking in to help because there isn't any code designed to call for it. If apps only utilize two cores, the quad core processor most likely won't perform any better than a dual core processor. The difference is that they won't have a contextual core and a language processing core to take any of the load off, so in this scenario, the Moto X has the superior hardware. And it's not something that only applies on paper. It's very clear that the Moto X outperforms both the One and the S4 in real life experience, all while consuming significantly less battery life. Speaking of which, is 24 hours of battery life a "mid-range" spec? I sure as hell don't think so.
I see no reason to not consider the Moto X a high-end device. It performs like one, the experience is a high-end experience, and in many ways, it outclasses other high-end phones on the market. You could actually argue that the Moto X is an overall better package than the S4 and the One. It offers similar to better performance, better battery life, a solid camera, a gorgeous screen, a solid and ergonomic design, and unique and useful features that other phones cannot even efficiently replicate due to a lack of dedicated hardware.