Hello jmpaul320. As for Eneloops, I am one who is very impressed with them. I have used them all through my house in various applications, including some flashlights that take AAs. Back when I was using my Mag 3D hotwire with the AWR hotdriver, I used 12 eneloops for an Osram 64610 50W bulb. I computed that the amp pull from that setup was 4.36 at 13.0 volts using hotwire rerating formulas. The light from the 64610 was one of the most pleasing incans that I have tried. In short, I observed that the Eneloops did very well. But, I did try to direct drive some 100W bulbs with the Eneloops, where the amp draw was 8 and higher. The lights worked, but the sag hit very early. My conclusion was that the Eneloops work reasonably satisfactory up to the 4 to 5 amp range, but not 8 or more.
Some time back, I used CBP 1650's, which were purported to be the best high drain AA's around at the time. I did not find they lived up to that in the 8 amp+ area. For the first minute or so they were ok, but after that the voltage sag really kicked in. I also tried another brand that was supposed to be pretty good (I believe they were Titanium Power Max 1800s....) but they were not all that great in my opinion either. Wish I could recommend a good battery and source for you, but for now, I am at a loss.
While you intend to just use the 458 bulb, you might give the Osram IRC 35W 64432 (should pull around 4.1 amps at 18.0V), or the Osram IRC 50W 64440 (5.1 amps), or the Osram 65W 64447 (6.4 amps) a try. I know this is CPF blasphemy to suggest backing off! But, to be honest, after trying all of those bulbs, including the 64458, I actually prefer the 35W 64432. Less bright? Yes. But, I figure at CCT 3400 (with 18.5 volts applied), it is still a very good looking beam. I calculate my 64610 was running not quite CCT 3500 at 13.2 volts, and it was very pleasing to me.
Trying out these bulbs might be a bit of fun for you with your setup. (After reading all these posts, it almost makes me want to go back and re-live my earlier hotwire days! I really like your light.)