I don't have any hard data. My impression is that Safe&Sound is simply Masonite's line of solid core interior doors. Instead of a hollow center (actually, foam filled), they have a particle board center.
As for steel doors, check out this link to the Green Glue door faq:http://www.greengluecompany.com/soundproofingDoors.php
It indicates that steel doors are comparable to solid core doors, as long as the steel door is 'solid' (inner honeycomb).
My question now: is a safe&sound door any better than a regular solid wood door from a sound transmission perspective? Home Depot and Lowes both stock solid wood (pine?) relatively heavy doors for almost exactly the price it would cost me to order a Safe&Sound.