I have an '87 305 TBI also and am getting about the same mileage, although I currently have a 3-speed auto tranny. I am going to get the injectors cleaned by these people:
http://www.injectorservices.com/, and rebuild the throttle body, make the fuel pressure regulator adjustable and crank it up a bit. I also plan to replace the oxygen sensors, plugs, wires, cap, rotor. Hopefully this will pick up a few mpg. Or I could just wait for you to do it and see how it turns out!
I think 16 mpg is very attainable and I've heard of people getting 20 mpg with your combo.