sure you can flower with CFL lighting, saves a bunch of money. but CFLs dont have the same penetration as a HID, a 400 watt HPS is really for small grows anyway so its reasonable to assume that by using 2- 250wt CFLs, placing them closer to plants, generating less heat, a similar yeild can be obtained. if your serious about growing i suggest at least a 600wt HPS though.
ps. make sure you buy a CFL with a 2500k spectrum for flowering.