I appreciate all of the great comments and thoughts in this thread.
TDC20, I hope that you didn't misunderstand my intentions. I am not trying to be argumentative, I was simply offering an alternative theory/explanation. Thank you for putting in so much work into this. Being from a scientific background, I am also inclined to "believe the numbers". However, experience has taught me that sometimes there are things that just don't work quite the way we think they do. This may be one of those cases. If the results we're seeing don't match up with the numbers, then we're missing something. The problem is, I'm not sure that we have established yet whether or not changing recoil springs really affects velocity spreads, or increases brass smileys. Unfortunately I am relying heavily on others' anecdotal experiences, not on concrete, reproducible data.
In order to make a valid comparison, we need to do some experiments with the same gun, same shooter, same ammo, on the same day. We need to limit our changes to the recoil spring only and eliminate all of the other variables.
Unfortunately at the moment I am locked away in a law school library and don't have access to a chronograph so I won't be of much help.