I have Lumia Black on my L520 and I can confirm it has Double Tab to Wake.
The rest of the features are the same from Developer Preview.
Hability to close individual tasks, rotation lock, driving mode, etc.
About batery, I haven't notice any change. My mobile with bluetooth and Wifi on during the whole day with minimal use lasts more than 3 days easly.
Performance wise, I can't really say anything. I bought a Nexus 7 2013 so I rarely use the L520 for anything other than calls and texts. However, it is still snappy as the first day I bought it.