would another 400w MH/HPS make a big difference?

Hi.
i would like to put another 400watter in the grow closet.

But would it make a difference?

I am planning on vegging 15 plants.

growing in a closet. (6feet in length,3 feet in width and 8 feet in height.)

Big difference is a subjective question, it would certainly give you a nice even light spread out across the area. If you got the money I'd say go for it unless you want to get a 1000 watter, the King of Lights..........

I currently have a 400mh over about 15-20 plants and find it a little less that ideal, the spread is smaller than i would like(although its in a deep parabolic reflector, the culprit.), a second one would be a perfect amount of light.