Caveat: I watercool pretty much for low-noise. I tend to dial down fans until they're unobtrusive (guessing under 18dB) and then look at MBM-reported temps, both idle and CpuBurn. I do overclock, but low-noise gets priority.
I've found, in my very limited experimentation, that plenum size (shroud depth, if you prefer) makes more difference if you're pushing air through a radiator, rather than pulling - so long as the "pull" side of the blades has no obstruction within, say, half a chord-span (otherwise you get interference with the fan - they're not so much augurs as wings so coanda effect is important - and can be easily screwed up).
I've also found that you can't hurt performance with too much plenum, and you may find additional benefits. For instance, if you use plenum on the pull side (where more doesn't seem to help temps), more does seem to reduce noise a bit.
Of course, this only applies to the extent that what you're doing matches what I'm doing - and your question was pretty telegraphic...
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