Nice!
Are you using a soft-start circuit on that FET?
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).