afaik, a space between the rad and the fans indeed provides better airflow. Feser claims that 27mm is the minimum for optimal fan performance.
(Not sure, but I also think they're more important for rads that have denser fin setups than low resistance ones like fesers and maybe thermochills.)
Edit: just read a short blurb on feser's forum that a triple feser rad showed 1.8C temp diff with/without xtender shrouds. Test details not yet out.

