Originally Posted by Rudeofus
No, I haven't done serious testing and, if you read my posts carefully, you'll see that I've left the possibility open.
However, the claim that Rafal refers to and that you reference above is found ONLY in the Ilford data sheet on processing black-and-white materials ( http://www.ilfordphoto.com/Webfiles/200621111117720.pdf ). Such a claim is NOT found in either the data sheet for Ilford Rapid Fix or Ilford Hypam.
What does appear in these two documents, however, is the differentiation between "commercial" and "optimum" levels of permanence. The references to Washaid in these data sheets only states that its use will speed washing. No mention of extended fixer capacity. I don't know who wrote which documents, but, to me, the B&W processing document seems simplified and directed at beginners. No mention is made of dissolved silver content in the fixer, nor of different levels of permanence. I would tend to think that the reference to extended fixer capacity is either an error due to editing or refers to "commercial" levels of permanence. I have never seen any reference to extended fixer capacity from Kodak, whose research I trust much more than Ilford's.
My point to Rafal is simply that two-bath fixing is proven and using sulfite to extend fixer capacity is iffy at best and would require testing (as you point out). I would think that simply using two-bath fixing would be a lot more secure if one were just interested in making prints.
On the other hand, if one is primarily interested in exploring photochemistry and, specifically, the ability of sodium sulfite to extend fixer capacity, then, have at it! I make prints for exhibition. I'm not that interested in expanding the database of photochemistry. I'll stick with the tried and true processes explored and documented extensively by those that have a lot more knowledge and experience than I ever will in photochemistry.