I am not sure whether Calumet has changed any of the Zone VI formulations since they purchased the company from Fred Picker. Fred stated quite openly that the fixer that he sold was plain hypo just like everyone else's. He said that his was less expensive.
On the subject of using sodium thiosulfate as the primary fixer...that depends on whether one is fixing film or paper. Sodium thiosulfate with the addition of sodium sulfite (32 oz -4oz) per gallon works very well as a paper fixer. In the case of film the recommendations are to use Ammonium Thiosulfate (rapid fixer) as opposed to Sodium Thiosulfate as the fixer. This is due to the iodine compounds that most modern films incorporate in the emulsion.
While there has been an exodus to TF4 in recent years, I have not found it beneficial to join the throng. I have measured the proportional stain using a densitometer and have found my stain density to be in keeping with what other TF4 users have indicate. Considering the cost of TF4 versus sodium or ammonium thiosulfate...it looks like a no brainer to me.
Just as others have stated, I have used Zone VI developer for a long time. I like it. I have discontinued using Zone VI fixer.