Just got the info from Nokia that Lumia 800 will be upgraded in the next few weeks. This upgrade should solve the Battery Peformance and WiFi issues, according to the information.
After the upgrade, you should get version 1600.2483.8106.11500. As usual, you will get a prompt on the phone when the update arrives. And as Windows Phone 7.5 (Mango) does not have OTA (Over The Air) feature, you will need to connect the phone to your PC and use the Zune software for update.