Watts per gallon rules only work when comparing same types of lighting to each other (ex.: fluorescent to fluorescent, LED to LED). They do not work when comparing LED to fluorescent due to the huge differences in energy efficiency.
The Evolve 8 light produces a peak PAR of 48 micromoles at the center on the bottom and has an average of 27.5 micromoles over the whole bottom footprint. At the surface the PAR is about 150.
A 20 watt CFL is going to be higher in PAR but I am not sure about anything lower in wattage.
I use the stock lights to grow crypts, anubias, some mosses, and a melon sword without issues. If you wanted to grow some high light stem plants (particularly those that turn pink or red under high light) you would want to upgrade the light.
Andy