Sorry cobra_red but im going to have to disagree. I've tested this mod comparison. Been at this predicament before.
I chose the shorties and my buddy bought the sct tuner with Tim's tune.
Both of us have 03 GT's. We have had exact same mods: SLP x pipe,
Magnaflow catback, Accufab TB and Plenum and our own RAIs....(except he has 3.73s)
- The tuner shifted the powerband lower... and made the engine feel overall more torquey.
- The shorties gave my car some high end power...but its still suffering in the low end without a tune.
He got the tuner and flashed his car before i installed my shorties. When we raced (on a closed course) and he only had the tune and gears over me, and it did not make the car pull that much harder over mine...He pulled maybe 1/2 a car to 3/4 of a car in a race from 5mph roll to 70 or 80mph.
Once i got the shorties on, there was a noticable difference in how much harder the headers made the car pull in comparison to the tune. Hands down, the shorties gave the car more power than the tune.
Of course, i am only arguing for which mod provided the 4.6L 2valve with more power.
I will say that the better bang for buck is the tuner, considering its only around 350-400 with a tune.
For the install work and price of shorties, its not worth it if. Plus like other mentioned, you can use that tuner down the road which will end up being a great investment.