If you bite on the desulfator plan, how about adding some science. It should really be quite easy.
1) All of the sulfate comes from the battery electrolyte.
2) A new battery that has been "completely discharged" or sulfated (theory) will have almost pure water as its electrolyte.
3) A desulfator that actually works would add sulfuric to the electrolyte.
Now, you can't easily tell how much sulfuric acid is produced by pH, but you can with a little chemistry. Take a battery that is highly, but not completely sulfated. Fully charge it. I would charge it, then trickle charge it for several days. Then, charge it again to get a little gassing.
Now, measure the sulfuric acid and soluble sulfate concentration. Titration with base is one way. Ba(OH)2 precipitation of all sulfate is another.
Now, attach the desulfator. When it is done, check to see if the amount of sulfuric acid has increased. If the things really worked, you should see a huge increase in sulfuric acid concentration from a battery that is 25% to 50% sulfated. That, of course, doesn't prove that the battery capacity has increased, but it would answer at least part of the question. You need to be sure that the volume of electrolyte doesn't change in the process.
John