I had the same thoughts as you. A sepic (buck/boost) regulator would have been the better choice, but also increased cost, complexity… and I suspect, introduced a myriad of unforeseen problems for the manufacturer. The limitations of maintaining regulation with the GT buck driver has also been discussed in the GT thread. You are correct, that the best performance of the GT will be realized while using cells that can maintain the highest voltage throughout the discharge curve. Keep in mind, that the discharge amps while using 8 cells in the GT will be about 1.5A per cell at max power. With that in mid, the Samsung 35E does a good job maintaining a high voltage while carrying a 1.5A load, especially for a 4.2V cell. Considering the availability of 4.30V and 4.35V cells, I was skeptical until I compared them against dozens of other cells using HKJ’s discharge comparator. Unless I missed something, the Samsung 35E is the better choice (and does not require a special charger to recharge them to max capacity).
Also keep in mind that our eyes perceive light logarithmically, and probably wont be able to detect lumen variations in the field while using any of the top quality cells that most of us already use.