There are many many things going on that can be making your phone heat up. It could be individual settings and conditions with your phone.
1) did you upgrade or did you full-reset the phone then install 8.1?
2) what apps are you running that could run hot in the background?
3) are you using facebook app? Have you disconnected the facebook integration from Outlook.com Microsoft account, which can help with duplicates and other issues.
4) Are you running GPS and Cortana all the time?
5) How long have you had 8.1 installed on your device?
Again, your situation and conditions are endless. Tell us something you know about what is going on. It could be some 520 are heating up, or more limited to your phone/setup.
To answer your original question, Cyan will be a firmware upgrade and will do tons to smooth out most of the hiccups and headaches people experience with 8.1.