The official announcement says it will be released in the 1st quarter of 2012. Most expect it in late January or early February.
Yes, you'll get a USB drive in the mail with the upgrade on it and also a SD card with new maps if you have the Nav option. Performance has been improved by optimizing the system's subprocesses and better use of the graphics accelerator. This has freed up bandwidth with the Freescale CPU for more critical tasks, reportedly improving the speed and stability of the system. There's a bunch of other improvements which are listed on the original press release.