I'm having issues with laggy performance after enabling 4G on my Evo. This is a common issue that is discussed in the Sprint Evo forums. See here:
http://community.sprint.com/baw/thread/51561?tstart=0
Sometime in April. Regardless of when or if the official gingerbread release is pushed to the Evo, the homebrew community will develop a custom ROM with gingerbread (and hopefully include 4G support).