Beach,
Whoever you hired as your Mustang consultant, you need to fire them....
The K&N is OK (Its EVERYONE'S first mod, if you want to call it that...
)
To be honest, you probably won't see any horsepower from it.... maybe 1 or 2 hp at the most...
The chip might give you a bit, depending on the chip it could shift your shift points and make them a bit more aggressive, HP gain is minimal on a stock setup...
Mufflers will only change the sound of your exhaust, if that's what your looking for.... no power gain from them....
Your first mods should be a conversion to 3.73 or 4.10 gears...
Best bang for the buck.... No improvement in power, but a noticeable improvement in ET...
Then look at some pulleys and\or a timing adjuster, and maybe an MAF...
Those should start bumping up that HP meter up....