No problems with mine although I mainly use 2x26650 (8.4V).
It draws more power and conversely sends more power to the LED array at lower voltages, as the cells deplete. I think thats the main attraction to using C-cells, you can run it at lower Vin, where its going to consume the most power.
DEFINITELY upgrade the +B spring. Even with the OEM driver it will draw up to ~6.5-7A at when Vin drops to around ~6.25V. Thats enough to anneal the +B spring, soften it so it loses its stiffness “K”. ALternately a strand or two of copper braid wrapped / woven around the spring will divert enough current from the spring coil.
I think theres some degree of variation across the BLF population though, because my light kicks into low voltage protection when the two 26650/18650 cells deplete to around 6V (3V per cell), and others have reported their lights running full-tilt at voltages lower than this.