Hi
I tested this out with a single black ice extreme, and tried it with and without fan shrouds too. For the life of me I can't find the original damn post I made about it, but the info is referenced (and discussed) here:
http://forums.procooling.com/vbb/showthread.php?t=10513
Basically the gains are so small compared to the massive increase in noise that it's just not worth bothering.