If you get a chip you wont lose any currently you have a speed limiter programmed into your computer that limits you to between 115-120 but if you dont get a chip or a speed cal you will not show the correct speed and you will be limited to about 90 MPH
You will lose somewhere around 1.5 to 2 MPG on the highway and will lose nothing in the city unless you have a lead foot or keep the RPMs above 3000 or higher
Engine life will not suffer. How you treat the car will. Change oil and keep the car properly tuned will go a long way in extending the cars life.
since you have an auto and a new model year auto a chip is the only way to fix your speedometer so it reads right, gears and tlok (4.10s) will hurt your gas milage if you do alot of hwy driving but if you do city or alot of city driving you should see an increase or no loss at all~ its well worth the gain in acceleration. I would consider buying a whole 8.8 rear end from speedconcepts.com (or .net ...?) 725~ plus 120~ shipping sounds like alot but add up the 7.5 up grade.... 400~ for parts 200 for labor~ and you are at 600 easy... so for only a little more you can upgrade to a whole rear end that will take anything you can throw at it and you can sell the stock one for a good bit or keep it for a spare just in case.
Thanks guys.Unfortunately I drive appx 40 miles each way to work each day.About 30 miles of the drive are 65/70 mph highway.But I don't have a lead foot and I think the performance gain will be worth it.I'm making arrangements to have a chip installed.
This is a great site,I'm learning alot.Mustangs rock.