I have a sneaky suspicion that the last update that the 810 will see will be GRD3. It's going to be much the same case as GDR2 though - where most of the changes are under the hood. We just don't have a device that takes advantage of enough of the new features to notice. Our screens are a lower resolution, our cameras aren't PureView, our 8GB of internal storage fills quickly... It just won't be all that awesome.
As far as 8.1 goes... I bet we will see GDR3 in February to March officially from T-Mobile. This gives them time to roll out the 1520 variant, mark down the 925's and move on to the next generation of devices. The 925 will see GDR3 right away - probably in time for Christmas. The 810 should follow in early 2014. This pushes the monstrous release of 8.1 to July/August of 2014 - or about the time that MS guaranteed support for current devices.
Now, here's the thing... 8.1 is going to be a big thing, not so much for the phones, but for WinRT. 8.1 will essentially merge the two platforms into one. Windows Phone will start supporting screens up to 10" and will run on more advanced hardware. It's the step we all wanted WP to make, but it wasn't quite ready. By July of next year, anyone with an 810 will be drooling over the newer devices and will hopefully have the urge to buy one.
Just my thoughts on the timing of it all...