I would expect that your AA-to-D adapters uses three AAs in parallel to replace each D cell? If that is the case, the voltage output would stay the same (about 3x 1.3v = 3.9v if using NiMH AAs) but the potential amperage that could be delivered is likely higher that the original D's. You would have to wire all 9 of the AAs in series to get close to 12v (at reduced amp current).
I would second the recommendation that you first measure the current amp rate of the light using the original configuration using both D's and the adapted AAs. It is very likely that the light is under-driver in its original configuration and you can get significant gains by modifying or replacing just the driver board to drive the XML emitter at closer its full 3.0 amp max current rating. Can you disassemble and post a photo of the original driver board? I would hold off the jump to a 70.2 emitter and upgraded driver until you also upgrade to Lion batteries instead of using NiMhs or Alkaline D's.
