I don't understand how load on the alternator circuitry will decrease mileage. I wouldn't think the alternator armature, and bearings are going to put anymore strain on the engine if you call for more current to run the extra running lights and brake lights. It is just going to spin at what ever rate the pulley turns and the diodes pass all the DC current. The diodes under load probably heat up, but I don't understand how that would put strain in the engine. An AC compressor, I can see straining the engine, and you can hear a difference in the engine when it turns on, but I never heard the engine stain from turning on my TT lights, or even when I transmit on my ham radio, which probably draws almost 10 amps at times. I may be missing something regarding alternator operation, and if anyone can help me better understand that would be good info to have.
I do agree that the LED's seem brighter then the standard bulbs, especially in rain and fog.
I did replace all my interior bulbs with LED's and I really like them.