I got thinking more about the speed-loss graph, and realized that it might be more widely applicable if the X-axis were changed to the ratio of sulfite to Phenidone. Here's the modified graph:
When the sulfite/Phenidone ratio is over 900, the speed-loss shoots up. When it's below 800, grain gets worse. So the range 800-900 is ideal. But is this true of other developers? XTOL was carefully designed, and I've found that .105 g of Phenidone substitutes for .2 g of DimezoneS. So XTOL's sulfite/Phenidone ratio would be 88/.105 = 838, which is in the ideal zone.
Note that speed (EI) increases as you go left on the graph. What happens if you decrease the sulfite/Phenidone ratio to around 200? I tried that in January by semi-accident, and got a developer that yields high speed and more grain. If you develop with it to a low contrast (i.e., under develop), you get normal shadow-detail (i.e., box speed) and wide dynamic range for high contrast scenes. Rudeofus: This is the low contrast dev you used a few months ago.
Anyway, this is something for designers of PC-type developers to be aware of.