I used to think that it is based on whether you outperformed your current elo in a match. But today I (Silver 2) played a match and got Match MVP in a lobby with 6 players who were Silver 3 and Gold but still didn't get any performance bonu...

Tried launching Val and it showed me as logged out and keeps saying "sorry we're currently experiencing issues with our server. Please try again later." Edit: working now! Thanks~