I too have used a 2 bulb screw-in fixture, and put power compact screw ins. the problem is the reflector is still crumby, so you're getting maybe 60% of the actual bulb's output into the tank. it's still a great idea though.
If you scope out my 15gallon tank in my sig, it's using one of those 10gallon, 2 bulb screw-in type fixtures. I run 3000K screw ins cuz I can't get cheap 6500K screw ins (our Wal-marts don't carry them, adn the
LFS want's $12.99 for one 10watt 10,000K bulb)
so it's a yellow light, but as you can see that tanks full of java ferns, crypts, and even some moneywort that grows slowly but surely.
I think I paid $15 for that little fixture, but it only has enough clearance for the U-shape bulbs...a spiral is too big, at least in the brand I got (which I think is top-fin or all-glass).
I think we've explored your lighting options