you can put the lights pretty damn close to the plants, if it's not too hot for your hand, it's not to hot for the plants.
as for yield with CFL's, i don't know yet personally although i've seen some amazing results from others. i myself am using CFL's because of cost, just can't afford to throw down on an HPS.
i must say that i like the CFL's too though because you can tweak your light spectrum and my own results have been much better than with the Mercury Vapor lights i used for my first grow, which was in dirt also btw. the plants need both blue (cool) and red (warm) light throughout the life cycle with mostly blue during vegative growth and mostly red for flowering.
and have you considered, investigated, DWC rather than dirt? come by and take a look at my imaginary garden and check out some of the other DWC threads, i think you'll like what you see.

anyway, toke amongst yourselves

bozo,
peace, love, dope