I'm trying to find some magazine tests with performance numbers on the GTS.
Several European magazines and MotorTrend have published GTS numbers:
Porsche 718 Cayman GTS laptimes, specs, performance data - FastestLaps.com
Scroll down to the main Comments section (not the sidebar) and click on links to image of source pages.
The best European GTS times (0-160kph/8.6sec., 0-200kph/13.6) and MotorTrend's 0-60mph/3.6, quarter-mile/11.9) are nearly identical to the best S numbers both Euro and American, also available in Fastestlaps. In fact the deltas, such as they are, favor the S by a tenth incl. a blistering 0-100mph/8.5 in a C&D test noting "without wheelspin would be faster". Unfortunately after testing every combination of base/S/Cayman/Boxster/PDK/manual, Car&Driver has not applied their rigorous test methodology to the GTS. I'd love to chew on the detailed data (incl. weight-as-tested, track temps, launch notes) in a C&D track sheet .pdf for the GTS.
How to explain these results? There were a lot more tests of the then-new 718 S, so allowing for normal variance between tests/cars it stood a better chance of posting an exceptional best. But its best times were not anomalies -- I've seen 11.9 ETs for the S with multiple cars/tests, and our own @Black718
did a certified 11.7x at Bradenton.
I think there is no question the 67mm compressor wheel GTS engine (+3mm over S) is capable of more power in theory, as it demonstrates in practice with ECU tuning. I wonder though if Porsche realized the S was conspicuously over-achieving for a factory-rated 350hp/3100lb. car; and a year later programmed the ECU to adhere more closely to the official GTS power rating (and not totally embarrass the dearer base Carrera). Such that the stock S and GTS have similar true horsepower something north of 365. And similar weight, thus yielding similar acceleration times comparing PDK-to-PDK.
Okay I now brace for a pummeling by GTS devotees....but along with pushing back on my theory, please provide an alternative explanation for the independent test data.