From my (limited) understanding, the GTS has a larger compressor wheel which should account for the higher overall HP. While the S doesn't increase peak HP that much, the area under the curve is pretty significant, so the gains through much of the rev range are much higher than 5%.
if you look at them they all have pretty significant gains until they reach their top point when compared to the stock baseline, which only added to my confusion. they all have similar increases on the curve yet the S still gets the least overall total boost. granted i an completely new to the "tuning" world. i come from the school of changing out engine parts to gain hp, not hooking up a computer
He once taught a german shepherd how to bark in Spanish.
When he drives a car off the lot it's price increases.
He once won the Tour-de-France but was disqualified for using a unicycle.
I don't always drive a sports car, but when I do it's a Porsche. Stay fast my friends.