Well that was a learning experience! I first shimmed up the XML with .060 worth of shims to match the SSC P4 height exactly. That is fine if you want to use the original optic, but I have to warn you, if you do you'll get a super bright SQUARE of light, just like the old Luxeon emitter put out, only bigger and much brighter! Since I'm not really into square beams, I decided I have to go with a reflector. The McR-20S reflector gave a beautiful beam, but with the emitter shimmed I had to put two O rings tucked behind the reflector to prevent it from touching the emitter with the head tightened, and that left a sizable gap on the outside of the head. I thought "well that sucks" and so removed the shimmed emitter and replaced it with one totally un-shimmed. Man that copper adhesive tape worked great, allowed me to peel that shimmed emitter out despite it being arctic aluminaed to the tape.
Once I did that, I only needed one O ring behind the reflector to take up the space the original optic rim would have occupied, and the reflector came down perfectly, just enough above the emitter so it doesn't cause a short but no farther. I suck at beamshots, all the heads look brighter in real life, and the new emitter is not greenish at all, but they do give you an idea of how much brighter that XML is compared to the Luxeon or SSC emitters. Tons of spill but comparing throw, I really don't see much difference vs either of the older emitters using the original optics, so haven't really given up anything with this conversion vs the older heads and gained wonderful bright spill.
If you are stuck on using the original optic, the SSC P4 is still the way to go as it eliminates the square beam, but if you can spare a reflector, then the XML is definitely a great option, as you will be amazed at how bright it is in one of these KX1 heads.
Original square beam KL1
SSC P4 w/original optic
XML with McR-20S reflector
side view, no gap!