And if you look at a torque diagram of a step motor you can see that the torque decreases with the increase in ppm value. Anyway, that's what I know
This is because the decrease in torque is from the higher RPM the motor is running at. This is different from micro stepping. Steppers usualy fall off pretty quick at a certain RPM, each motor is different. Microstepping does not increase the desired RPM, it simply takes a step and divides it by the given value resulting in higher resoloution. Say you have a motor, 2.8 degrees per step x 200 = 360 degrees. Now, micro stepping x 8 it would be 1600 steps per 360 degrees. Either way, with the motor running at the same RPM, it should have the same torque. I think you are confusing RPM with PPM.
I have sent you a personal message.
Brett