I've got several different (but in some cases connected) questions and if you don't mind, I'll try and address them in one response.
pbm ~ DEXRON(R)-VI in a Toyota that calls for T-IV:- It's hard to comment(which is not terribly helpful to you) but in my view, having tested the T-IV and the WS, the WS appears to be a backward step from T-IV. I would say that T-IV was closer to DEXRON(R)-VI than WS is, but I'm sure you'll appreciate that when a company like GM builds their own fluid, they're not taking into account other OEM hardware, such as friction materials, for instance. I don't know if that helps or leaves you with the same dilemma!
Eddie ~ I do not agree with the so called 'Universal ATFs' either and agree with you on that point. Anyone that knows anything about the various OEM approvals would instantly realise that many of the Universal ATF claims (if not all) are completely ridiculous and in some cases the marketing material is so bad (due to their lack of knowledge) that they can't even spell correctly! Doesn't inspire much confidence, does it!
JHZR2 ~ Your comment is a valid one and an interesting point of debate. DEXRON(R)-VI was developed to be fully backward compatible in GM Hydramatic transmissions. In previous years, many other OEMs had taken advantage of the extensive testing carried out by GM and just simply "bolted on" extra requirements to suit their own needs. Needless to say, when GM decided to move forwards and develop DEXRON(R)-VI, the primary focus was what GM transmissions require, both for now and in the future. Obviously GM are not going to carry out validation testing in other OEM applications that are not relevant to their requirements. In theory, you are correct in that if the recommended fluid was appropriate for DEXRON(R)-II, III, or any of the precursors, it should be good for DEXRON(R)-VI. The problem is that in saying that it is not known whether those OEMs made other changes to, say, friction materials or control systems that may or may not allow that assumption to be made. I know it's a 'woolly' answer, but there just isn't a simple yes/no.