Steps/unit never changes. When you change the kernel, you need to reset your accel and velocity settings.
As you increase the kernel speed, you're taking time away from the CPU to actually run windows itself. So as it gets faster, you may start to see stability issues in Windows.
Many people have reported smoother running motors at higher kernel speeds, but the author of Mach3 has always recommended using the lowest kernel speed to give you the speed that your motors need.