I would still consider adding a cooling fan to help extend the life of the bulbs.
Considered, and kind of required I think. I bench ran the light for twelve hours today to make sure I didn't have any overheating issues, none came up, the ballast stays kind of hot but I can keep my hand on it as long as I want, it's maybe 100-110F.
The bulbs do run pretty hot, if I don't run them with a 1" stand off above the glass the amount of heat built up makes them too hot to touch. The entire hood, with stand offs gets "lukewarm" to the touch. After 10 hours the bulbs get hot enough that I can only keep a finger on them for about 4 seconds.
With a fan running blowing the in the general direction of the hood, not even right on the hood (a 4" clip-on fan) there is a noticable drop in temperature, so I'm going to run a fan blowing across the hood of the tank. If anything it will keep the light from heating up the tank as it's only 20 gallons.
Here is a final pic of it running on the tank with the "custom" stand offs installed (they are chop sticks cut down and painted black.)
I added the plants today, the guppies are sure enjoying them.
Also thanks for the clarification on 2x vs 4x. I put a amp gauge on the AC line of the ballast and it is drawing 0.76 ampers. So in theory it is producing 91.2 watts, but I'm sure some of that is lost in heat.
However, if it is indeed producing that wattage that gives me 4.56 watts/gal, so I may need to adjust the wiring. We'll see how the plants do.