Any advantage in running a UBEC w/ 6cells?

[BrianG]

Not really. It won't hurt, but won't help that much either. As the supply voltage (battery) gets closer to the output voltage (6v), the advantages of a switching UBEC become much smaller.

If you are drawing an average of, say, 1A on your servo (and this is an extreme figure), you'll lose about 1.2w as heat. And it will be less than that because servo loads aren't constant. This equates to about 83% efficiency, which is close to a switching UBEC efficiency of about 85%.