I'd always say start with exhaust... flows are definitely my favorite. That sound is just
... Everyone has them, but who cares... everyone has a mustang as well...
Next thing people will probably tell you is gears... Will make by far the biggest difference for the money. Everyone swears by 4.10s, some by 3.73s. Probably comes down to how much highway driving you do. Tons, I'd go with the 3.73s, not much... 4.10s.
Oh yea, shifter, definitely the shifter. Get the Steeda Tri-ax. Once you get it installed you'll wonder WTF Ford was thinking with the stock shifter. It belongs in an F-150.