I have several DTS CD's and I would say they definitely sound different than the CD's. It is more about a new way to experience an album that you are already familiar with. I have "Fly Like An Eagle" by Steve Miller. Doesn't sound like a DVDA, but adds a different take to the album. I have a few others and I would say the same about all of them. Can't imagine there were too many made. Really not sure why they exist, since most car stereos can't play them and they are not rippable. Why not just make it a DVD?
I do have an Alpine DVD Nav that can play them mixed down to 2 channels and they just sound awful in my car.
I don't know the technical difference between a DTS CD and a DTS mix on DVD, if there is one. My Porcupine Tree DVD's DTS tracks sound awesome. I think, like everything else, it just depends on the competence of those making the decisions.
A DTS CD just gives you an "alternate take" on how to experience a familiar recording. It is not a step up in SQ. A well recorded CD still sounds pretty good. The DTS just allows a change in presentation. It will sound different.