Updated drivers only applies to the Lumia devices, as Microsoft manufactured the Lumia series via their subsidiary Nokia. If you don't compare a phone to PC's, you're doing it wrong.. they're just small computers (always have been). Windows 8 for instance, doesn't even bother much with more than generic drivers unless there are special features involved (and yes, you can install drivers for the GPU on any device if the OS allows it, it just so happens that most phones and tablets don't because they are systems-on-a-chip that are manufactured for an individual phone/tablet only and are typically loaded with very user-control reduced Operating Systems). However, a snapdragon phone can typically use the same drivers/firmware as another one in the same series so long as the OS is the same. I'm not trying to hammer anyone in particular.. but I can't stand error that is purported as truth =\. Device driver | Define Device driver at Dictionary.com. Drivers are hardware and OS specific. Computer - Wikipedia, the free encyclopedia
As for the developer preview, there is indeed a newer version of WP 8.1 available (WP 8.1 update) but that doesn't mean you necessarily have it (or that either of your devices don't have it). =P Just like Windows 8 computers don't automatically have Windows 8.1 just because it's available, you either have only 8.1 with the update, or you don't have it at all if the versions are the same build. That said, as I mentioned in my last post - the OS version is always the same if we are talking the same OS version.. the drivers are only relevant so much as the OS actually uses them (which on integrated devices isn't much for most things, hence why firmware updates are so important). Drivers are a half-step between firmware functionality and OS functionality. They are largely efficiency routines anymore (which as your contact in MS mentioned, would make sense for improving performance.. not so much stability unless the old drivers were faulty though).
You can't install your own GPU drivers in Chromebooks that doesn't make them not computer does it >.<? The Motorola Razr I uses the same CPU/GPU as some netbooks, notebooks, and tablets.. does that make it not a computer because you can't install a driver? No.. it doesn't =P, it's a computer just like notebooks and tablets are computers. Tablet is a form factor, not a type of device just like Smartphone, Notebook, Ultrabook, Netbook, Desktop, Workstation, Server, Mainframe, and Supercomputers are. They're just different shapes and implementations of computers. What you are referencing is a limitation of the Operating System on your phone, not something that relates to what it is at all. If you really wanted, you could install Android on your phone (you can install the same Windows on the Razr I as you have on your desktop if you really wanted.. although it would run like garbage XD).
I do this for a living, servicing and supporting thousands of devices with all kinds of shapes, sizes, functions, and purposes with a wide variety of software betwixt them all. I'm not trying to be condescending.. I'm trying to provide useful, accurate information. Smartphones are the epitomy of a PC (Personal computer | Define Personal computer at Dictionary.com) as they fall exactly into the definition of what a PC is. Phones don't allow users to install drivers and such.. because carriers have closed networks, and allowing users to modify the OS on their device may jeopardize or compromise more than just their own device.. there's literally no good reason to allow users to do that with their devices as far as carriers are concerned. Phones are limited by the software placed on them and that's about it. The same hardware runs tablets (some of which also run the same Operating System your notebook or desktop computer uses). Companies that make hardware don't differentiate cell phones from other form factors. Their hardware is typically categorized for mobile/portable devices (smartphones, tablets, and notebooks/ultrabooks/netbooks), desktops/workstations, commercial grade, or industrial grade. All of the form factors I mentioned above use ARM, X86, or X64 architecture CPUs that all do the exact same thing in different ways. The only noteworthy difference is the level of capability each option has compared to the others. 
I suppose in some ways you are technically correct about T-Mobile, although mostly just because they have the most hybridized network (using WiMax, CDMA, LTE, and other options all at once.. but that's also part of why their network isn't in the same realm as ATT and Verizon) ^.^. So I'll give ya that one Tidbit, touche! 
