I've been running Phantoms since they came out. I have both 1000 watt and 400 watt versions. The 1000W can be dimmed down, but I never dim bulbs, and have never used the feature on any of the ballasts I've owned. I used to have Lumatek ballasts, but never used the feature on them either.
I was told by a rep for Hydrofarm that there is a major spectrum shift when you under power bulbs. I'd have to talk to someone up on their physics, but he said the gas doesn't get as hot as it is intended to get, and while it may look similar to your eye, running 600W through a bulb designed to operate at 1000W , does not produce the spectrum that it puts out when running as intended.
This is just my opinion, but if you need to dim your bulbs, you are better off running your bulbs at full power and figuring out a new way to deal with heat.