I think your issue, simply put, is due to the weight-to-horsepower ratio of your boat. The G-Series is a great boat, loaded with standard features. The addition of the Sea Legs and extra battery is the equivalent of carrying 3-4 extra passengers.
My 22 footer with 2 logs and a black 115 ran 27-28 (gps). I added an express tune and fiddled with the transom height. My top speed was... wait for it... 27-28 (gps). Much better handling, but top speed never budged.
Expecting more from that heavy of a boat and a 115 (of any color) is a pipe...
I don't think your boat/trailer combo is as heavy as you describe, but fuel, water and gear does add up fast. I think 5K# is a maximum figure. Do heed the ramp advice from the reply above. Know your ramps... spring, summer AND fall.