The wise and wonderful people at CPF suggested I try a small modern optic instead of relying solely on the vintage reflector. The choice of sub 20mm optics for the XM-L is quite limited. The best I could find was this 16.1mm 37º optic from Ledil (This 23º optic would probably be better but wasn't stocked by either Mouser or Digikey). It arrived from Mouser today. I had to snip off the mounting posts, but it otherwise fit perfectly over the XM-L's PCB, attached firmly with its own adhesive tape:
|Ledil 16.1mm 37º optic mounted on XM-L|
|Beam profile of Cree XM-L with Ledil optic compared to original incandescent bulb|
|Beam profiles of Sturmey Archer lamp with different light sources|
With the Ledil optic I'm willing to declare my Sturmey Archer LED upgrade a success. There's even the possibility of upgrading to the 23º optic if it ever makes its way over to North American shores.