Dilutions are often used to control both the shape of the curve/contrast and sharpness. Many people use XTOL at 1+1, for example. 1+3 can be useful too. So you might need to test the "capacity" of your developer's basic stock solution. ie how many ml of the stock solution are needed to develop a roll of film (~80sq. inches). I may have mentioned this before but perhaps some type of test in which you determine the minimum quantity of stock solution needed to develop an entire role to "D-Max" at the development time you have been using to plot normal contrast. I put "D-Max" in quotations because it is not likely the film's actual D-Max. So maybe a reasonable test could be:
1. For a given film, determine the highest density you get in your single-frame tests with normal development time (may require some alterations to your standard testing since you are not currently plotting the full curve). Maybe instead of plotting a curve you just give one frame "full exposure", develop for the normal time, and measure the transmission density.
2. Give an entire roll the same exposure as in (1) with normal development and measure the resulting density. This would have to be repeated with decreasing amounts of stock developer until a decrease in density is observed.
Another series of tests that could potentially be worthwhile would be "pressure tests" on agitation frequency. How tolerant is this developer to decreased or increased (ie Jobo) agitation frequency before getting uneven development and/or streaking?
Take this for what it's worth though, Mark. I'm not exactly in PE's league when it comes to this stuff. I'm just raising a few points that neurotic people like me would think about before using a developer (in addition to the other tests you listed).