According to Wikipedia:
Tungsten halogen lamps behave in a similar manner to other incandescent lamps when run on a different voltage. However the light output is reported as proportional to voltage to the power of 3, and the efficacy proportional to the power of 1.3.
[13] The normal relationship regarding the lifetime is that it is proportional to voltage to the power of −14. For example, a bulb operated at 5% higher than its design voltage would produce about 15% more light, and the efficacy would be about 6.5% higher, but would be expected to have only half the rated life.
That being said, a 5% over voltage will decrease your bulb to halff life.
And according to this
site, at 10% above 6v, your bulb will last only 50% of the rated life.
You mention a peak voltage of 8.5 volts, however the RMS of the AC voltage is still only 6 volts in which case that value is what is important to a 6v halogen bulb. Whatever above the 6v produce by the generator happen to be, that extra voltage will shorten the life span of the bulb. Let say the generator produce a 6.6v. That is 10%(if my math is correct) which means the bulb will last only half the rated life(according to Reflectalite) but will be brighter by probably 30%(according to Wikipedia).
Personally I never played around with generator for a bike but I have looked into it. So I cannot be 100% certain this info is correct but at least it looks good
