Suspension! Lol that's what I just bought fir mine
SUSPENSION! Tweak that handling. Right out of the box the mustang feels a little clumsy. Lowering and bracing will reduce body roll and thus increase your launch and acceleration around corners. A good coil-over kit and subframe braces are a good place to start.
I also have an 08 and I went with exhaust then intake and tune and I was plenty satisfied but now I got the bug and cant stop I'll be droppin the stang here pretty soon and who knows what after that lol but whatever you do decide you can't really go wrong!!
If your drag racing I would not suggest lowering I say if it's a stick car bmr upper control arm and lower control/relocation brackets if it is a automatic the the c&l racer intake it makes the most power with the sct tuner just what I suggest but do what you want