Yes, both under Mach 3 and under manual control I've been able to change Spindle speed with various S commands. I confirmed that a S value issued matched the rpm shown by a handheld tach under both Mach3 and manual control of the VFD (NOWFOREVER ES100). It's just in Mach4 that the Spindle runs at a constant 850 rpm.
My index is on Pin 10 any my Gecko 540 assigns the Spindle to Pin 14.