"...mechanical bits are sourced from 10,600RPM Savvio parts..."
Looking at their website, Seagate have now lumped their "slower than 15,000" parts together as "Enterprise Performance" branded rather than Savvio.
As all the ex-Savvio 10K.6 drives are self-encryting, I'd suspect that the bits are more likely from the older 10K.5 range. A ST9900805SS with a cache hike and some magic flash pixie dust would fit the bill and would be a three-platter drive if so.
It would seem that IBM have let the cat out of the bag as there's not a hint of the flashed-up version at Seagate. I wonder if this beastie is only supposed to be a production prototype for priviliged customers, with the "official" launch version being the Savvio 15k based part you hint at?
 Also, given how long the Momentus XTs have been around, that would fit better with what was on the shelf as a base for an Enterprise version once those had been seen to be a Good Thing.