Not only is it safe to use 2 li-on 8.4V input, but its fully regulated and you get the same OTF...out the front lumens.
Surefire LX2________________Stock__________2 Surefire CR123__________249.2______1 sec_____376______9,400_______
__________________________________________________ _____________ 245.4______30 sec________________________
__________________________________________________ ______________ 243.1______1 min_________________________
__________________________________________________ ______________ 239.2______2 min_________________________
__________________________________________________ ______________ 237.7______3 min_________________________
Surefire LX2________________Stock____________2 IMR 16340____________251.5______1 sec_________________________
__________________________________________________ _____________ 245.4______30 sec________________________
_________________________________________ 497mA at tail___________ 243.1______1 min_________________________
__________________________________________________ ______________ 240.0______2 min_________________________
__________________________________________________ ______________ 238.5______3 min_________________________