As stated in the opening paragraph of the Measurement article, CO2 is the most difficult parameter to apply, to measure and to get right in a planted tank. Have a read of the article =>http://ukaps.org/forum/viewtopic.php?f=34&t=467
A Drop checker cannot tell you what the specific needs of a given plant is. It cannot tell you what is physically going on inside the reaction chambers of the plant's photosystems. It cannot tell you what osmotic forces are being generated at the leaf surface, how quickly the CO2 is diffused through the leaf or how efficiently the enzyme reactions and transport mechanisms are working. It can only give you a rough estimate of what the CO2 value is at a specific location in the tank. Based on this limited capacity the data from the checker is just as valid whether you have high, low or medium lighting.
I think where you might be getting stuck is that you assume that a green drop checker should be valid regardless of lighting level. This is not really the best way to think about it. At lower light levels the low energy of the light throttles down the demand for CO2 and nutrients. The speed at which the enzymes capture and transport the CO2 molecules does not have to be very high and so a marginal CO2 concentration in the water column can work. We see this behavior in low tech tanks that do not even have CO2 injection. You could therefore get by with minimal injection and a near blue drop checker indication. As you increase the light, the speeds of the reactions increase because more light energy particles per second are making contact with the chloroplasts. In order to satisfy the increased number of reactions caused by the increased light energy a high concentration of CO2 is required so the injection rate must be high enough to drive the pH in the drop checker to green. As you increase the light further the rapidity and volume of the photosynthetic reactions require an even higher availability of CO2 so the injection rate has to increase to the point where the acidity drives the dropchecker into the yellow. Therefore a single injection rate and a single drop checker coulor does not satisfy all possible lighting levels. The higher the light, the higher the injection rate needs to be which is reflected as a brighter color in the drop checker. Higher concentrations are necessary to "force feed" CO2 across the leaf surface to keep up with the sheer volume of photosynthetic reactions occurring within the chloroplasts.
Terrestial plants, or aquatic plants in emmersed mode have breathing holes called Stomata which allow air inside the leaf. The CO2 concentration in air can be on the order of 150ppm so they are more likely to be nutrient limited than CO2 limited. In water the solubility and availability of CO2 is much lower. Other factors such as water flow and temperature have a much greater effect on this limited availability. The only way to overcome these difficulties is to increase the concentration to compensate. Te green dropchecker color is just a rule of thumb which more or less indicates an average concentration of 30ppm. In most cases this is sufficient but using extreme lighting pushes you completely out of the "norm" envelope. In fact 48 watts T5 on this size tank may still be over the top but it's a lot closer to normal than 96 watts.
Hope this helps clarify.
Cheers,