Watts per gallon is a long gone myth much the same as the one inch of fish per gallon "rule".
There's a lot more that goes into lighting than just the bulb, there are three main components: the kelvin temperature, the height of the tank, and how well the light is reflected into the tank. That whole thing bundled into one is known as Parabolic Aluminized Reactor rating or PAR rating. They use it for car headlights too ^_^
So for you, it'd be easy enough to find 6,500K or 6,700K CFLs at Walmart and replace them in your hood. The 13 watts max is for each side, not combined so you can do the 9 watt ones they have in the fish section if you wanted or there are other watts if you go to the actual light bulb section in hardware. Home Depot and Lowes never had bulbs above 5,500K so I wouldn't bother with them.
Something else you can do is make sure that the plastic between the light and the water is always nice and clean so the light can penetrate well, and then you can even use aluminum foil in the back of the bulbs to reflect the light into the water better. Lastly, you can paint the back of the tank black or put some sort of background on it so the light is reflected back into the tank rather than just disperses out the side/back.