When you add the salt you change the chemical make-up of the developer. When you change the chemical make-up of the developer, you essentially end up with two different developers.
Because of that, you must insure that the negatives themselves have the same final contrast from both developers, in order to make a meaningful comparison.
It would be even better if you could contact print the negatives next to a 21-step step wedge to compare tonality and range.
I'd also like to mention that using standing development can be a blessing and a curse. If you ask Steve Sherman, possibly the world's expert practitioner on the topic, he will tell you that standing development is a great method. If you see one of his prints some time, I am sure you can be convinced that standing development is an alternative. But I do not see him using Tri-X at EI 25,000 either.
What DOES happen when you use this technique is that you change the tonality of the film quite severely. It is extreme compensating development where shadow values are brought up and highlights toned down, and when you print in the darkroom, I sometimes find that the tonality can be really strange from these negatives, and I think this is where some of the conflict surrounding whether it works or not comes from, possibly even contempt from darkroom printers who think scanning film is like cheating. Just a wild guess.
What the bottom line is in this thread, however, is that HP5+ will never be a fine grain film. There is no developer that will make it a fine grain film.
Embrace that grain, because it's beautiful! Last year I used about 25 rolls of 36 exposure 35mm HP5+, in replenished Xtol mostly, and the prints look really great at 16x20 inches. The grain? Just find some interesting subject matter, and make meaningful photographs, and you won't even see it. Photographers are their own worst critics, and nobody else cares about grain.