It will perform much better, so much you won't believe it's the same car. 2.76 really kills the torque.
Nobody can tell you the change in mileage, you change the gears and then tell us.
For an extra $72 a year in gas just go for it. Since you have 27" tires then go for the swap. If you had 25" tires it would be too much gear for freeway.