Digital ballast question

I like the fact a digital ballast can dim 600w bulbs but is it true if you dim a 600w bulb to 400w it will lose 50% light output????
also can a 600w dual spectrum bulb be dimmed or is it just mh/hps bulbs?
see I have a 600w dual spectrum light kit but ideally wanted use a 400w mh bulb for veg to save on electricity bills, am I just better off getting a 400w mh kit or getting the digital ballast?
its the sunmaster fx 600w dimable ballast

I have a magnetic and digital. only thing I like about the digital ballast its light half the size looks cool . but outher then that . save your money get the magnetic ballast. you will never dim the lighting. why would you ? digital is expose to save electric. , I beg to differ . electric bills are calculated by kilo watts. so no matter what your using 600 watts. about 30 bucks a month , yes you can dim any bulb with the digital. but you wont, you will be learning you need more lights then just 1 lol later on. so get the magnetic ballast save 200 or 300 bucks, you will need so much more later . like a good dehumidifier, I push those, mold is a evil monster , no matter what you chose have fun !

for got to say if your new get florescent for veg its good for seed starting cloning , and veg the mh is only good for veg, its grows a tad faster ,

I beg to differ kind. I have turned mine down and it has saved some grows by less heat.
There are some other benefits I can't think of. I went with the advice of growers that were producing.
Check it out then make your decision. There is a lighting thread,should be some info there.