I use the Engergizer Ultimate Lithiums in my XL50. No issues whatsoever. Somewhere I read/heard that even though the voltage is around 1.78 unloaded, it drops to a level consistent with alkalines when put under a load; therefore, the initial voltage is a non-issue. And that the lithiums will stay flatter at the loaded voltage longer than an alkaline. Plus it's nice that the lithiums are about 1/3 lighter! Does anyone else concur?
I've only used EA92's in my XL-series lights for testing because the standby drain becomes costly with lithiums. I imagine that the XL200 will benefit even more from lithiums because it draws about 900mA, but most of the time I use NiMH because it's cheap.