I think Doc summed it up well, too.
I've mostly used a G2L or Polytac LED at 11:00 out at the end of a 12" rail. On my SBR, when it still had a FSB, I had moved it to 1:00 and reached across with my thumb, but found that awkward. It was a compromise because I could put my hand further forward, without pushing the light so far out it was longer than the barrel. I've since tried the light at 12:00 after shaving the block and putting on a longer rail, and with an X300 i think that is about ideal (not ideal with a TLR1, though).
I don't know anything about the IR capabilities of the lights so I can't comment on that aspect.