As prompted by @mcjtom I tried adding alu tape to the head of my IF19:
Did not change much regarding beam profile or brightness… Don’t have any good pics, but not much difference imo (left is before, right after).
However it did add some weird petal-shapes to the spill, probably where the light reflected by the alu tape is getting back into the TIR, through the 3 “legs” of the TIR. Rotated the light a little between the camer so you can see them moving along with the light:
Bottom line: Would not recommend doing this on a IF19.