While the company I work for modifys only American made cars, from 4 cylinder to V10s, we have found that increasing the horse power and torque not only provides more performance but typically gives a slight increase in fuel mileage. Vehicles that come from the factory tuned to operate on 87 octane fuel typically gain the most power as we tune them to run on 91-93 octane fuel. By running on 91-93 octane fuel, we can advance the timing and typically lean the air fuel ratio at the lower rpms. Under normal driving this equates to better fuel mileage.
On the 6.0L 2004 Ford diesel pickup, we can get 150 more HP and 225 more lbs of torque! We here from customers all the time who are getting 2-3 more mpg while cruising at 70 mph with the program.
As for shortening engine life or effecting the overall reliability, unless you drive the vehicle harder than you would with out the chip, you're actually extending the life of the motor.
Look at it this way.
If you have 300 feet to get to 70 mph, a higher hp/torque motor won't have to work near as hard as a low hp/torque motor will, when in the same size and weight vehicle.
The main thing you have to worry about with aftermarket chips is the air fuel ratio. Most standard chips are designed to be installed on basically stock motors. Normally an air intake and cat back exhaust are ok. But, put headers or a big throttle body and you're most likely going to be running too lean. Running too lean causes detonation. Call it pinging or valve rattle. If it's ignored, you may end up with a burnt piston.
The more power you have, the less you have to press the gas pedal to get to lets say 60 mph compared to a lower hp