The way I understand it with carbon filters is, that you want to replace the air in your closet about 4 times a minute to keep the smell of your crop in check. Your grow space is 48 cubic feet. You could use a 200 cfm fan and a 200cfm rated carbon filter to scrub the air before you blow it into your attic. I built my own cool tube and run a 400w in a space thats less than 2 feet x 1.5 feet x 5 feet. with with a 200 cfm fan and filter and I cant smell my cabinet from the room its in. I can hear it a little but its not too bad.
If you bought a larger fan, you could run a 600 in there no problem. just buy a fan speed controller so you can slow the fan to get the temps dialed in.
How many 5 gallon pots can you fit in the closet? Id put 2 plants in your closet and veg the plants until they are about 1/3 your maximum height train them to make them nice and bushy. I have 4 in my cabinet and It is too crowded. But i grow my plants screen of green.
Check out the grow in my sig.