I love digilux bulbs.
That being said, i don't think the dim able feature was intended for permanent use. Its great for 1 day at 50% 1 at 75% 1 at 100%. But there is more to Bulbs then simply how much power is being pushed through it. There are gasses, etc inside the bulb that were formulated to run at a certain power level and its inconclusive how well bulbs ran at different power rates perform. Dont expect performance to be exactly 50% or 75% just because your running that wattage.
This is a good point, my 600w Lumatek with a 600w Horti changes color temp when I kick it down to 400w.. Still runs like a 400w HPS but it's got more blue @ 600w
I think the bulb should be matched for the wattage run to get the best results.