I don't think you'll have much luck driving the motion outputs of the plc from Mach. The high speed functions only work when you are running a motion program from the inside PLC program. When you are using them as normal outputs (which they really would be in this case) there is some limitation as to how fast they can change state. The motion control program that resides in the PLC runs along side the normal PLC I/O function and runs somewaht independent of the normal PLC I/O Scan. I have seen normal PLC scans of 500-600 per second, where you are looking for output frequencies in 100,000 times a second.
A normal PLC runs kind of like this - Read the inputs, go through the program logic setting the outputs, write the outputs. The Micrologix PLC's I used did this about 500 times a second. There is some other overhead in the process to check for errors and such...