There would never be enough extra strain to cause a drop in mileage, the pump almost turns effortlessly.
I have over 100,000 miles on my MITUSA pump, the filter in my kit is after the pump, therefore any debris from the pump wouldn't get to the injectors. As for the return, mine goes back to the filler neck (major PITA) it chokes off the filler neck, making it take fuel very slowly. You can tie the return into the supply line from the tank. ( the purpose of the return line is to regulate pressure. via a check valve with a spring and shims.)
The true beauty of the MITUSA pump is that it feeds the CP3 after the FICM allowing you to give lots of lift pump pressure without blowing the seal out of the FICM.....I run my pump at 20psi...which is actually better for the CP3.
You can also tie in a shut off solenoid on the return line with a Hobbs switch so at boost it will let the MITUSA pump go up full pressure (around 80 psi).
My kit does have the "fuel log" as well, its only purpose is to allow fuel to bypass the MITUSA pump if the belt brakes, I have never broken a belt.
I did also have to "Clearance"the TCM cover the pump shaft can hit it when the motor is under torque.