Realistically, what can I expect as far as MPG and HP is concerned when it comes to adding an aftermarket muffler?

 

Not mucking fuch... maybe a teeny bit of power & mileage. Mostly sound, possibly throttle response because it may be breathing a bit better.

 

You will defiantly get more attention from the cops.
I get around or under 1 mpg with a cold air intake I ended up taking it off and going back to the stock box.
I'm thinking about cutting a hole in the bottom of the box and adding piping down by the bumper.

 

I added a glasspack with cat delete. Noticed a SMALL difference in mid range speed (say if your at bottom of 3rd gear then you punch it). and throttle response was a little better. Other then that,meh its ok.

If you have a tbi system,best bang for the .25 cents is the quarter mod (stick a quarter on top of the spring in the fuel regulator) and grind down the ridges around the injectors. then advance your timing 2-4 degree's. I really noticed a difference after that and running 89 octane.

 

Remember that these engines require a certain amount of back pressure unless you get the PCM tuned to adjust for a more open/free'r (<--- is that a word?) flowing exhaust.

Google says no, it's free flowing - cest la vie

 

Headers is the edge of a sword..... One slip and you go into the I like the sound, vroom vroom vroom and lose mileage. But on the other side you can drive it normal and maybe see about 1-3 mpg increase....