Difficult to answer really because It depends how shaded it is when the sun isn't on them.. if it's dropping below 200 umol then they will stretch and it's sub optimal tbh. which is what I find happens in general.
If you live somewhere nearer the equator the power of the sun allows you to get away with more.
At least at 3-4" under the cfl the babies will be getting around 300umol consistently which is what is advised for babies, but it will obviously be inferior to the 1500+ umol from direct full sun.
I'd stick with your cfls..