Excluding Juice
No other one mod is gonna give as large of a performance gain you will feel as gears
As a matter of fact, gears will make a much greater difference than ........
several other typical mods you see peeps do all the time
Do you have a concern about that mod
Now ... Excuding Gears
And considering you are still close to stock ..............
Lets talk Exhaust
and
Lets exclude sound and talk performance
You could change out the entire system and won't gain a lot

The biggest bang for the buck here is the oem 4 cat mid pipe
The pipe will give more than headers or catback
Intake is good for a nice gain ... TFS, Edel, oem Cobra
Mods that give bad gains for the money spent IMHO
TB
Maf
Rockers
Catbacks
Ignition Systems
Off the shelf pcm chips
I got a small gain I could feel when temps were cool with my CAI
It was a great gain for the buck spent as it was home made
My pulleys gave me a nice torque gain ALL across the rpm band
HEY ...........The wisest thing you could do IMHO is ........
think about how you want to improve your Stang
and
come back later for more specific answers
by
giving more specific Q's
As you can see ..............
I gave you answers which pertain to making more performance
A shifter is great
but
It don't make any power
See what I mean about being specific
Grady