I converted a Maglite Mini to LED a few years ago, and was just okay. Single bulb, Maglite brand, dropped right in.
As for multiple vs. single bulb LED flashlights, I will say that we use the Streamlight Survivor at work, and they use a single bulb (they also use 4 AA batteries and burn through them pretty quickly). They are BRIGHT as HELL. I don't know quite enough about LED technology to say for sure, but I suspect the number of bulbs isn't the only factor that determines brightness.