I measured mine with and without the hood using the output of a solar cell. For me, the improvement was definitely worth the time and money to make the hood. If I didn't have the tooling or the confidence to build it, maybe not. To each their own.
You're bang on about the small plants: I've been flowering under CFLs for a few years now and use a bunch of small plants in cut off 2L bottles and get a whole bunch of little popcorn buds. If you're looking for the tall colas you see in the pics, CFLs aren't the way to go. I like those PLLs - they might make better side lights than my CFLs.
Ital, another thought about your box: I supplemented mine with a little
50W HPS and noticed an improvement in the size and density of the buds (everything else stayed the same). That one I linked to is a replacement for commercial lighting and the bulb fits a standard E26/27 base rather than a mogul socket. It might be an option for your small space - I think it was about $60 and came with a lamp, ballast and capacitor. You'll need a lamp holder (about $2) and a hood of some sort, but even the flat reflector you've already built ought to be fine. After running it for a full flowering cycle, I can honestly say that the ballast generates way more heat than the lamp itself. If you mount the ballast on the back like bozo did with the PLLs in the pic, the heat should be more than manageable with your 120mm fans and you'll be getting 2100K rather than the 2700 of the CFLs. Now that my hooded array is done, the next job is to get the little HPS back in there...probably inside the hood in the center lamp holder.
Are you planning to paint or mylar the inside of the box eventually ?