Re: 2008 gt
I would start with exhaust, it wont add much of anything (on the 05+) but it makes it sound much better. So you get some power and a much needed boost in exhaust note. Brands vary but most like the ford racing mufflers (little round ones) they are the same as the borla brand for MUCH less. Around 250-350 for axle backs.
A good cold air and a good tune (not just a mail order can tune from whom ever...) will wake the car up a good bit. Most like Bamachips.com for their 05+ but many companies can do tunes, just make SURE they are good (ask here). Some see 20rwhp with a tune/CAI combo or more. Runs 700~ bucks for this on average.
Lowering the car is cheap and easy and makes it look great (no power gain but makes driving much more fun and the car looks like it should stance wise from the factory) it can be expensive to do this as you need CC plates with the springs and an alignment afterwards.
Other things to look into, underdrive pullies, aluminum drive shaft.
2003 Cobra Vert (Redfire) #3938 of 5082 @ 05/27/2003
472rwhp/493rwtq -Modification List
- Dyno Sheet
2012 Mustang 3.7L M6 (Kona Blue)
2011 Ford Edge Sport (Red Metallic)