Somewhere there is a thread on BLB bulbs in which Sandy King mentions measuring the consistency of the light output. He found a good bit of variation over the initial period of time the bulb was on (first minute IIRC). And after that, it varied some but much less. At the time the consideration was for alt process and the conclusion was that the variation after the first minute was not enough to be concerned about with the long exposures.
In this instance, I wonder if the instant on bulbs have the same kind of light output variation, and I wonder how well the output is stabilized. Someone with the proper measuring apparatus will have to check, I guess.