Well, both Ian and PE: I fully know that there is a considerable change in activity with that much baking soda or carbonate. I do not have (is it litmus paper?) or other material to specifically measure the pH but there is, of course, considerable activity change with either. Maybe it is simply my false perception here with evenness, but I did (many times this) put two pieces of film, vertically, into two developers, one normal and the other with both carb and baking soda (enough to even out the activity back to normal) and I have to say that the initial evenness of the carb & soda was considerably better. Of course, at the end of development I think that this mattered little. But it was, for me, interesting. (I keep trying to find out things that Kodak 'forgot' to explore when I do these crazy things!) Besides there really are advantages to the baking soda that the 'literature' rarely mentions. Slowing down activity without resorting to either dilution or colder temperature is surely worth mentioning.
Folks, always take with a grain of salt my 'finds': I am not a chemist or one who even has a profound theoretical base for these assessments, but, nevertheless, they might be interesting to consider. - David lyga