I seriously doubt it will hurt. Just don't go to big. If you go too big it will hurt your low end torque, that could cause a little less mileage. In the big picture since you are keeping the stock manifolds it probably would hurt that much. If you have any mods planned in the future I would definetly go with dual exhaust or a larger single exhaust.
Any properly set-up computer controlled injection will give you better mileage over a carb. It might not give you better performance depending on the set-up (in most case better perfomance can be had too) but, this will depend on how you are measuring performance. The advantage to fuel injection is it can adjust as you drive to give you the optimum air/fuel ratio. You can't get that with a carb.
My 79 camaro got much better gas mileage with just headers and glass packs than it did after we put pipes and mufflers.
My wife had a fuel-injected Geo Metro (5spd) and she was getting about 40 mpg, then she hit a rock and exhaust broke off right in front of the cat. She got well over 50 mpg, the car sounded *almost* cool.
My truck's mileage went up about 3 mpg when I dumped the rusty tailpipe and muffler for a glasspack, although it has gone down now that my stock manifolds are falling apart.
The whole point of the story is that giving the engine a free-flowing exhaust system will increase mileage so long as you don't get headers with such big tubes that all the low-end torque is gone, because then you'll have to floor it just to start from a stop.
Also, a free flowing exhaust system makes your car sound better, which makes it feel faster (in addition to how much faster if may actually become).
I mean, a slow car that roars really awesome (like my truck) can feel way faster than a fast car that's real quite (like my daytona) simply becasue if you floor it and the car doesn't make any noise, it just doesn't seem like it's doing anything.
I say the bigger the better. when it comes to duals.Just insist on the "DR. GAS X" pipe to scavenge!!!! It is amazing!!! Use small diameter headers for small engines, then, go big from there aft!
2 penny`s worth.
Have fun today.
Slider in Wa.