The UF-T90 originally came equipped with the HX-1175b driver and were stock at 7 amps. I would think it should be fine resister modded up to 10 amps with 2S. You may want to post the large diode that is rated for 10 amps max. I have driven MT-G2 with 4D cells into the low teens with the driver without issues, but the MT-G2 is one tough emitter. I don't know if the SBT70 has the same toughness.
A couple threads on the driver:
Review: UltraFire T90 SST-90 3x26650 Flashlight from Wallbuys (Picture Heavy)Review: UltraFire T90 SST-90 3x26650 Flashlight from Wallbuys (Picture Heavy)