Exactly. Like the man at my local hydroshop says - the more power the better.
A dumb man once said, "If an ounce worked, than two ounces will be twice as good"
And of course he says that, YOU"RE BUYING FROM HIM, HES A SALESMAN NOT A RELIABLE SOURCE ON HORTICULTURAL IDEAS...maybe he knows a thing or two..but he is a salesman first...thats how he pays his bills and builds his grow room.
At 67 watt/ft2...there is NO need for more lignting. At 6 feet in an enclosed space a 400 HPS is more than enough penetration.
Adding another 400 will only cost you 200-300 dollars, plus hooking up ventilation, taking time away from your grow, buying new fans, cutting new holes in the box.
Most plants cant even utilize that much light without CO2 injection(save for some light intense equatorial varieties that wont even fit in a space that big!) and in a small space like that CO2 injection will likely get sucked out of the box before it can do any good.
Side lighting with REGULAR Floro's would be ideal for your setup, if you REALLY WANT(there is no need) more light. T5's cost more than twice the money and only give you an efficiency increase of 25% to maybe 40% if you buy the really good stuff. A 7$ shop light fixture mounted to each side plus regular floros that are chosen to even out the spectrum(blue colors would be ideal considering your HPS resides mostly in the Orange-Red range) would be more than enough light added to your setup...but it sounds to me like you're just being wasteful.
At 67 watt/ft2 you're doing just fine with your lights. If you really want to increase your yield look at other areas...Temperature(you say is fine), Medium(hugely over looked factor), overall system, Nutrients, Bloom Boosters, or increase efficiency in your space by doing a SCROG or SOG.
If you really want to increase your yield. Do a SCROG and use a simple CO2 injection system and put the input under the screen and wait to turn it on until you flower. Then every bit of CO2 that you inject will have to pass through the screen(aka the leaves), allowing most(at least more efficiently than normal grown patterns) of it to be absorbed before it leaves the box out the top.
My overall recommendation..SCROG that bitch, add CO2, maybe add side floro's lights(ONLY if you add CO2, otherwise its a huge waste of money and time).
I know this went down two years ago now...I'm sure you, personally, have learned much more in that time...I'm posting this in case anyone else finds their way into this thread like I did.
