Thanks for the info on intake/exhaust ratios. I'm not sure how much drag the reducer's going to add either but I don't think all that much with the low flow fan. The fan is 4" diameter and I plan to mount it straight to the coupler, thinking that would be the least amount of backpressure?

Yes the intakes have light traps and the closet door will be shut all times as well, unless I'm actually looking in on the plant. Wasn't planning on the exhaust having one as there's no windows or anything in the attic, just a small vent about 50' away that never really get's direct sun.

The shoplights are just an idea, I can toss smaller CFLs on the sides instead if you think heat might be better controlled that way. The colors can be changed, I was planning on throwing them in for flowering.