I've never seen a 300 watt CFL, and with the amount of light you get from CFLs, I don't think you would need anything that puts out that much light.
I put 26 watt CFLs on the outer walls on each side of my garage. They do a great job of lighting my yard. I feel that they put out as much usable light as the high pressure sodium lights the city uses to light our streets, and at a far lower cost. I have no idea what size the City's HP sodiums are, but you can bet that they consume a lot more power than my florescent lights eat.
I bought my fixtures at my local Menards, and the fixture has a built in photocell in the light so they take care of themselves day and night.
