Hi,
I have an 865 EFI SM from May 2008 on, and compared with previous 865 carbed engine I was able to largely try for more than two weeks, it's a different thing, but just at start and during the heat-on time.
Once it's at right temp, no massive differences are detected.
About MPG, there is a slight difference fovouring the EFI one, but nothing dramatical.
Compared with old 790 Carbed, there is less high punch favouring very low revs grunt.
Note: EFIs do not longer bluish the exhausts...
Cheers!