Someone can send to martin
koolance's 140-120mm adapter to see if there is any difference because of shroud effect in cfm (or martin can cut out himself such from whatever flat material). I'm guessing though that in real world such 'flat' adapters would hurt actual real cooling because to addition of GT's big motor hub blind spot one would get additional blind spots near sides/corners because of adapter.
Martin: btw, other 140mm fans are not exactly shroud-less aswell, afterall, SR1 has rather uncommonly deep (compared to other rads) shroud integrated in it, at least from one side..