Sounds to me like you still have some loose, dirty ground wire connector problems. Being an 87 renix, they are nortorious for this. Also sounds like the ground problem, or short, or crossed wires, may be in the TPS sensor wiring harness between the ECU and the TPS, or in the TPS harness ground wire. Also check the O2 sensor wiring harness and sensor wires for shorts, they frequently get too close to the exhaust manifold, burn and short, either of which would cause your 10 mpg problem. Also, check the the O2 sensor and TPS sensor connections, connector pins themselves as they get dirty and loose!!!!!!
The 87 renix can handle running colder and still go closed loop even at 100 F, and still get excellant mileage so don't worry about the thermostat for now, but you might see a small (like 1 mpg) improvement in mileage with a 180 F thermostat.
also, Are you sure the manifold leaks are sealed now??????? Could one of them been warped, and thus still leaking even with new gaskets?
Also, have you checked the compression yet to see if you have any bad cylinders. Be sure to check it cold and hot to see if any valves are leaking at just one temperature extreme.
Lastly, you should be getting a much narrower 2.0 to 3.0 volts on the O2 sensor at say a constant 2,500 rpm after only a one minute warm up, even if the coolant is only 100 F. The Renix (87-90 models) will switch to closed loop at very low engine temperatures. If you are not getting a significant narrowing of the 0 to 5 volt swing at 2,500 rpm within 60 seconds of a cold start, something in that circuit is bad. Mine runs 1 to 4 volts at idle and about 2.2 to 2.7 volts at 2,500 rpm and oscilates back and forth across the sweet spot (2.45 volts IIRC) once every second to half second.
Lastly, not sure exactly what, where, you measured from, but if it was a ground sensor wire that read 0.05 to .8 volts, IT IS YOUR PROBLEM!!!!!!!!! Just splice in a new ground wire from that sensor ground to the battery negative post and it might solve the last of your mileage and hesitation problems. It solved my variable idle speed problem on mine!!!!