There are two codes to turn THC off in mach3, they are listed in one of my other threads about THC control.
To use them I had to create custom macros, I think I used M10 and M11, trouble is my machine is at work and I can't remember what was in them but it is listed somewhere here in one of my threads. I think M10/11 did not cause a delay or glitch in mach movement.
To use the macros I created two code snippets in sheetcam one off and one on, these were then called from various cut rules.
It all worked well but the new system has made much of it pointless as it is completely different and has it's own on/off codes.
I will see if I can find the codes...