The new MBox 3 Pro features all new converters over the MBox 2.
Black Lion Mod states the following:

"The Mbox3 Pro is Avid’s redesign of the old consumer-grade Pro Tools interface, the Mbox2, but now with Cirrus Logic CS4272 converters, which feature 24-bit/192kHz conversion and a dynamic range of 114dB (the same converters used in the Apogee Duet and Ensemble).

So in terms of just the converters I would say the MBox 3 Pro is probably doing a pretty good job.

Kirk