Hey, thanks a lot for the 5.1 backup (and thanks to the OP as well). This saved me a lot of trouble updating to 5.1. Hardest part was just waiting for everything to download, though it helps I'm not new to the adb game and I already had twrp installed.
Some others have reported they lost their recovery even though they left it unchecked during restore - I left mine unchecked and didn't lose it, so not sure what the inconsistency there is. I went ahead and flashed the updated version of twrp afterwards anyway.
I did have to re-root but that is simple on this device (unlocked). As for 5.1 it is a big improvement on the 8.3; while I know rebuilding the cache/etc. explains some of the speed/"new" feeling it is still much faster, something I already knew having experienced it on my N6. The GPE edition still competes with the best of the current crop of similar size tablets out there, so it's too bad they stopped selling it.
I tried a Tab S 8.4 (which just went on a massive sale in the US but I got it before) and it has a much nicer screen in some respects, but everything else about it is terrible. When I found out the 8.3 got 5.1 I pulled it out of the drawer and went about trying to remember how to un-root and update it, which led me here - again, thanks so much, because it looks like it is a total pain in the ass otherwise. With 5.1 the 8.3 beats the crap out of the S 8.4 in most other aspects and the screen is still pretty sharp, just not quite as nice as the formers, but who cares
The Tab S is going back today.