I've got a stealth 6D Mag that I used to run a 64440 IRC off 5x IMR26700, 20.1Vbulb on a fresh charge (now rocking an EFR 15V/150W ellipsoidal MR16, aka Burnageddon, Scourge of Newsprint), and I couldn't get the 64458 to run nice with my lightly-modded stock switch; flashed bulbs on full batteries, and I never got around to NTCing it (and bleeding batteries down was just too much hassle).
Even though I started on a C Mag this is pretty much my first complete and what seems to be a pretty reliable light.
I am using Jimmy's 2 resistor circuit with an NTE 2395.
So far I have had good luck with the bulbs but I typically give it a little warm-up blip before clicking the mag switch on.
I have a 6D mag I want to try w/ 6 cells but the 24V 6457 HLX is a bit larger and does not seem to work as well in the FM reflector.
The only hassle seems to be using the new mag switch. I made a small PC board that slides into place but I could probably come up with a more robust switch contact.