Yes there's at least one thread with some info. Another source could be 2GFusions since they're not just limited to 2G Sports. Both revisions are the same SYNC 3 core, v3.4 is just updated and has a different set of themes. New features in v3.4 are simple things like the 'calm screen' (which just shows the time and no other info, but you have to go into the settings to turn it on since it's really made for newer models), and some other options for features that we don't have, and vehicle settings can be changed on this screen rather than on the IPC & steering wheel controls (again, nearly all of them don't work in our cars). Really, the only point to do it is the new looks.
Which Ford Performance boot screen? The GT one was taken out, but the ST, RS, and Raptor animations are still there - plus all other animations. If your car didn't display anything other than the Ford logo, that means the changes you made didn't apply, or the value didn't point to anything valid. If you used the test menu, that is a non-permanent change.