a mixed spectrum beats using only 6500k or only 2700k though...
What most people do not realize when speaking in terms of color temperature(6500k, 2700k, etc.) is that these bulbs are NOT monochromatic. Under a spectroscope, a 6500k and 2700k bulb look strikingly similar, with each having a red, green, and blue peak. The difference between the two is which peak is bigger. So even if you use all 4 of the same tube, you're covering a roughly full spectrum with either 'bloom' or 'veg' type tubes.
To truly take advantage of the T5 fixture, You should choose a bulb with the largest red and blue peaks and smallest green peak. These bulbs appear pink to the eye, and are commonly sold for freshwater aquarium use. Not all are created equally however. Some, such as the AquaMedic Planta have a very desirable spectrum when viewed under a spectroscope, while others such as AquaticLife's freshwater bulb lack in some area(specifically far red in this case). Bulbs like these will outperform the usually recommended 6500k and 2700k tubes for both veg and flowering. The major problem with them is cost as they usually run $20-25 per tube. A few bucks more per tube is worth it to me if the plants can more effectively use the radiation supplied.
Either way, if you are going to use T5 fixtures, the best favor you can do for yourself is invest in parabolic reflectors. If your fixture has bulb spacing of 2" or more, replacement reflectors are available for ATI fixtures that are easy to retrofit into other brands.
Sorry to ramble on, but bottom line is T5's work best with pink tubes and individual reflectors for each bulb.
Happy Growing!