The EasyWhite XM-Ls are set up like the older MC-Es and SSC P7s in that there are four dies on one board that are either all in series or 2 parallel strings of 2 in series. They are universally less efficient than their single-die counterparts due to heat buildup on the MCPCB. If you look at the CREE product characterization tool, you'll see this. Also like the MC-Es and P7s, they are much harder to focus. Even with very large reflectors they will be floody, and they have a tendency to have dark crosses and/or donut holes in the middle due to the separation of the dies. While they are easy to wire up, unless there's a big reason to have a single unit, you're better off getting multiple LEDs and better optics.

If you have a large enough standlight you don't need a doubler. A voltage doubler does decrease your maximum brightness at the expense of giving better performance at low speeds. Ktronic did a comprehensive series of speed vs output tests for various configurations on MTBR and pretty conclusively showed that there are very few situations where doublers are useful. If you're using XM-Ls (of the single-die variety), they have such a low forward voltage at 500ma that you'll be able to get good light out of them even at moderate speeds. I run 2 XM-Ls on my dynamo system with 2 20mm Carclo Tight optics (10003). I'd upgrade the optics to something larger, like the 26.5mm's (10048), and a XP-E Red-Orange on the rear, and its pretty consistently bright.