With math like that I am not surprised this approach appeals to you. My original statement is correct: 100 BTUs in (feedstock); 85 BTUs out (useful energy).
Period.
--Boris
The point is that it is 100 BTUs of waste material (not useful energy) being run through a process to turn it into 85 BTUs of fuel (useful energy). Given that the waste material is waste, that's like putting rocks into a blender and getting oil for the energy needed to run the blender. 15% is incredibly efficient. FYI, normal chemical-to-electrical power plants lose about half the energy during the conversion.
That feedstock is otherwise useless crap that would actually take resources to dispose of. You arent putting 100 btus of USEFUL energy in.