I've had okay growth with the default light fixture. It will be slower, obviously, but you won't need to worry about CO2 and frequent water changes. I've grown crypts, even giant hairgrass (I can't yet figure why it grows like crazy in there, but not in my 3+wpg bigger tank!). Even Echinodorus tenellus pygmy chainsword grows ok.

I would think retrofitting with a 13W CF would be a pain in the ass, and would also get pretty hot as well...