I've been running 1 pair of 4ft T8's. They were obviously an improvement over the stock 15W lights, and I even started building a really nice hood for them.
But recently I upgraded to 4 bulbs, and I regret it. My plants do not seem to grow any faster, the overall brightness/contrast has not changed - to the eye. My water has also started to become more cloudy. Wether the water cloudiness is a issue with the lights or not, I don't yet know.
I also have an 10 gallon experiment tank running same hours same plants similar ratio CO2, but it has 2 screw in CF bulbs, and the plants in there grow 2x faster, water is crystal clear, and plants just look so much healthier. I only fertilize 'K'.
I have a feeling that these regular flourescent bulbs just don't quite get "bright" enough - to make the grade. Don't get me wrong, I get the impression that you can have 1000 watts over your tank, but if it's spread out, and thin, the plants just don't seem to enjoy it as much as a more concentrated source.
So, if my opinion fits in the corner of your mind, i'd also like to hear your feedback on your system for comparison. I'm shopping AH supply's 2x55 setup...