Nick, Claire, what can I say but that the developer formulas are not correct for a number of reasons. If you get good results, then more power to you. Knowing what the formulas lack, and the potential for bad results, I will not use them myself.
The old Darkroom Techniques magazine, precursor to Phototechniques if I remember that correctly published not only formulas but sensitometric curves from several different C41 developers. Those curves showed the variations that you might expect from the different published formulas. Given that, then the color reproduction and image structure also varied, but no one ever reports on those, and they are what make some of these films truly stand out.
I wish that test equipment for granularity and sharpness was available to us. Then there would be fewer people looking for the magic bullet out there, or using guesstimated formulas.
I have personally run several different color developers which work with one film, but fail with another color film and the same goes for papers. A given formula may only work 'right' for one product. This is for 2 reasons. 1, the manufacturer tests all of his products and most competitor products in his developer and tailors the film to meet release specs in that developer and 2, the home experimenter who publishes formulas cannot test all films and cannot run the exhaustive tests that the film manufacturers run. These tests include sensitometry, color reproduction, granularity and sharpness and also may include hardness and development sentitivity (to time, temperature and agitation).
The only formula published out there that seems satisfactory is the RA blix, but even that can be mixed up incorrectly if the pH falls too low. Below about 6.3, the blix can begin to cause dye stability and hue problems among other things. You see, the final pH of color products is critical to those two factors in most all cases.