That dimming light might work fine - I've never used one so I can't comment on it. I use HO T5 fluorescent lighting for my clones from the first day they are cut until I start them on 12/12 for flowering. This saves on heat generated and the plants seem to veg out just fine - just have to make sure there is plenty of light so the plants do not stretch looking for more light. For flowering I use a 400w HPS light except in summer when its too hot to keep things cool - then I use the T5 lights. Yield is down some but thats to be expected. And in case you don't know you can use the HPS light for vegging if you like. I have a switchable ballast light system with MH and HPS. I see little difference except that (from what I've read) the HPS is a bit more efficient for the given amount of light produced and produces a bit less heat. While the HPS light does get quite hot I've noticed the MH light seems to get hotter.

I also use the same size grow tent as you do. There isn't enough light with a 400w light to fill the tent completely - something I knew when I bought it. I also knew I'd have to make room for a 560cfm fan and the ducting it requires - which is why the larger tent. The light seems to cover a 3x4' area pretty well.