yep... with 400W HPS in a 2 by 2 closet you NEED to get temps down.. also it is very likely that the light is drying the air out.. like badly.. so I think at a bare minimum you need a good intake and exaust system (not just moving air around inside) and consider the humidifier option.. heat transfer does happen better when the air is wetter.. thats why on a hot humid day it feels much hotter, or on a cold humid had it feels much colder.

You could set up the intake and exaust with two $5 fans.. it doesnt have to be expensive but you do need to get temps down.