I had a 13' Riverhawk(very similar to the gheenoe). Could not put a big enough motor for below the dam. Worked okay for protected sloughs in the reservoir though. I would think 15'6" model would be minimum size need for down below (mostly due to having the ability to have a larger motor). Bass boats don't throw much wake, the big cruisers watch out. Often 1648 jon boat will work just fine and is easier to find for sale.
As far as kayaks go, I would look at the Jackson Cruise and Cuda 14. Both allow stand up fishing and have plenty of speed to move around from spot to spot. The 115 is tad small for moving around the lake. 135 would be better, although I like the Jacksons better (and they are made here in TN).