@ CC, when you cut them down like that do you use a glass lens in front of the optic? I like to use glass cause it protects the optic from getting scratched up, would doing so make it so you could turn ~1mm (thickness of lens) less off the pill threads?
If the glass lens is touching the front face of the TIR it will affect the beam pattern. If you can leave an air gap between them (like the raised lip of the white plastic holder on the 20mm 10* single TIR, you know?) then it doesn't screw up the beam, but it doubles the optical losses each time the light passes a barrier - front face of the TIR is one barrier, rear face of the glass is another, and front face of the glass is a third barrier... don't do it just to protect the TIR. Sometimes it's necessary (like in the Roche F6, the way the front bezel o-ring seals there's just no other option than to re-use the glass lens) but avoid whenever possible.