Ideal Qualities of a CD/SACD Transport

There are many discussions regarding the relative performances of DAC but, comparatively, very few address the capabilities and performances of CD/SACD transports. Having decided on the Bricasti M21 DAC, I now have decided to upgrade my system further with a new "shiny disc" transport. At present, I have a Wadia 8 (30 years old and CD-only) and a Theta Digital Compli (20 years old and two-channel SACD as well as CD -- it also has an excellent internal DAC based on Theta Digital’s highly-regarded Generation VII DAC). As best I can tell, both function perfectly. However, in the past couple of decades, transport technologies have improved, and I want to get the highest performance (within reason) that my new digital system can provide.

Thus, I propose the question: what factors, qualifications, capabilities, etc. are most important for a high-end CD/SACD transport to provide? There is no question that they are pricey. The "bargain" of the group is the PS Audio Datastream, and it lists for $7,000. The prices move up steeply from there, with, for example, Playback Design’s MPT-8 at $20,000 and the "entry-level" dCS Rossini one at $26,500. Others are even more expensive.

I have three additional questions: (1) at what price point does diminishing returns really start to become apparent; (2) several companies that manufacture both a DAC and a transport highlight their proprietary I2S connectivity -- thereby permitting the direct transmission of an SACD’s DSD layer (instead of first converting it to PCM) from the transport to the DAC. Sonically, how big a difference does this make?; and (3) also, is there a sonic difference from a CD’s datastream being transmitted to the DAC via I2S versus AES/EBU or coaxial?