I would shoot for something neat looking. I would put something over the tub (some kind of plastic top or coated board that won't absorb water) that will hold 2 net pots for 2 plants equally spaced apart. Build a screen over the entire tub and do a bathtub SCROG. The 400 is probably a bit too small for the space but, it will work. Get a large air pump (or a bunch of small ones) in the tub with a bunch of air stones (the more air the better). Hopefully the tub is made of porcelain and it will keep the res naturally cool. Put a carbon filter in the bathroom and leave the bathroom fan on for half of everyday to bring in new air and there you have it. Easy and cheap.

Maybe you could use the leds for cloning or veg? I don't like to use anything but HID lighting just for the efficiency.