Hood,
A few questions for general understanding.
When you select a kernal speed, should it be such that it takes into account a worst case of all the axis's running
at some total pulse rate for a desired feed rate of each?
The Kernel chosen will dictate the max speed you can get. If you have a Step (pulse) per unit of 5000 @ 25KHz the max you will get is 5 revs per second(300RPM) at your motor. This is for each axis so no worries about it being shared

Is it correct, that with the SS generating the steps and not MACH that the kernel speed of MACH is not so important?
When using the SmoothStepper the kernel in Mach should be set to 25KHz, this is for no other reason other than if set to anything else the plugin shows wrong feedrates in Mach. The feedrate is correct but just shows wrong, set to 25KHz and it displays fine. Other than that the kernel speed has no effect on the SS as all of the pulses are produced externally from the computer.
When talking of "kernal speed" is this the rate that Mach can function / take priority in the Windows enviroment with out something interfering with it's ability to generate pulses? Maybe I'm just asking for a definition of kernal speed.
Its simply the max frequency that the pulse engine in Mach will work at.
Hood