The price feed varies very little between one witness to another. If you mean when when do the revision, the price feed before and after the revision are the same. Only the constitution of the index varies. The algorithm compile the new index composition and built it with the last price recorded.

The memo key is the same as the account "announce" so everybody can see on the blockchain what is happening via Cryptofresh

Cryptofresh Hive account@announce : http://cryptofresh.com/u/announce
Cryptofresh Hive account@bittwenty.feed : http://cryptofresh.com/u/bittwenty.feed

The memo key is only used by witnesses to be able to read the data we send in the blockchain so they can built the price feed and adjust some parameter. You wouldn't need it to burn shares if you decided to.