Blackberry are taking steps in the right direction as the Z3 has only 1512mb of ram.
That being said yes windows phone 8 and 8.1 does work pretty well on a 520.
I had a 520 and for the most part it works well. Except when you open an app go out of it and return to it there is the dreaded apps resuming dots that happen very often.
Also the best apps and games are unavailable to you because of the lack of ram, while some devs manage to slim down their code to work on low memory devices such as the 520 most do not.
Blackberry 10 does require more Ram yes but it also performs better and more consistently.
Sent from the ocean using my waterproof Z10