One advantage of using cfl's is that they produce MUCH less heat than a traditional HID bulb. This means you can place the lights closer to the plants and the plants get more light. I've grown with pretty much the same lights you've described and I could get them as close as 1-2", yes inches, from the plants with no heat stress. Find a comfortable distance for your lights and plants. I'd lower the lights to about six inches from the plants and then hold your hand near the highest part of the plant. If your hand gets uncomfortably hot, then it's too close for your plants. If your hand isn't hot, then I'd say it's a safe distance.
As far as is this enough light to grow your plants, well it's all about lumens baby. Your lights which are 23 watts, are commonly labeled as "100 watt equivalent" which means they put out as much light as a 100 watt incandescent but only consume 23 watts of electricity.
1600 lumens X 3 = 4800 Lumens. Your growing area is 18 square feet, but with two plants I doubt you're using all of it. I would section off a part of your closet and cover the walls with mylar or some other reflective material to get the most out of your lights. For Marijuana you want at least 3,000 lumens per square foot, but 5,000 per square foot is ideal. If I were you I would invest in more lights. You can buy a Metal Halide fixture if you want, it would greatly increase your yield over CFL's, but they cost more to buy and run. If you're looking into an HID fixture, I'd recomment Hidhut.com, thats where I got my HPS from and they seemed to have the best prices when I shopped around the net.
Figure out your total lumen output and divide that by your square footage. Aim for the 3,000 mark.
Marijuana plants need airflow. A fan in the closet would circulate the air, and also make your plants have stronger stems. You can pick up one a 12" oscilating fan for under 20 bucks. This would provide a bit of airflow, and help your plants to make stronger stems. Even though the bulbs don't produce a lot of heat, in a small enclosure it can still heat up.
Shopping List: More CFL's, try to get "daylight spectrum" or 6500K or 5500K or cool white. All of these basically mean the same thing, good color spectrum for vegetative growth. Get a fan, and maybe check prices for HID lighting online and see if you want to make the investment.
