Pretty much identical? It what way? GTS has 50 more hp and weighs about 200kilos less. Trust me the difference is a lot. Own an s3 and drive a cayman s most weekends areas where an s3 would touch 130mph, Cayman doing 150mph. First hand experience argue all you will...
Sorry to p*ss on your bonfire, trust me, the difference is NOT a lot, just look at the maths and the physics below, argue I will....
Audi S3 weighs in at 1415kg (S-Tronic)
Cayman GTS weighs in at 1375kg (PDK)
Audi S3 has 300ps (296bhp)
Cayman GTS has 340ps (335bhp)
Audi S3 has 380nm between 1800-5500
Cayman GTS has 380nm between 4750-5800
Audi S3 has a 0-62 of 4.8 (S-Tronic)
Cayman GTS has a 0-62 of 4.8 (PDK)
Both cars will run down the 1/4m strip in around 12.7secs
The Audi S3 weighs 40kg more and is around 40bhp down on the GTS but the S3 has a far wider torque band that will make up for this small power and weight difference, this can be seen in pretty much identical 0-62mph times and 1/4m times (0-110mph)
In real life though, the S3 is easily tuned, the GTS is not. The GTS has no tuning potential unless you're willing to splash out 5 figure sums on a forced induction conversion.
In real life, the S3/Golf R will be mapped and pushing the Cayman along, in a straight line anyway.
The S3 is far more practical (Can carry a family) and is far more useable (4wd traction, year round performance) It is more economical, cheaper to tax, insure and run (no Porsche tax on consumables) Porsche 2yr warranty vs Audi/VW 3 yr warranty. the GTS is £58k, the S3 is £28k, quite a difference.
You talk of the Cayman S (Which has less power and less torque than the GTS model) doing 150mph and the S3 doing 130mph at the same point, I would suggest you are talking nonsense. The maths and the physics do not stack up.
Have you seen the acceleration figures of remapped Golf R's and S3's, 0-60 in 3.7, 100 in the 8's and 1/4's in the 11's. No Cayman will get near that, I think even the new GT4 will struggle to break any of those times. The S3 will be able to repeat those times launch after launch after launch by your granny, the Cayman will need a professional race driver to get those times, average Joe may do them 20% of the time. Ever tried to launch a rwd car?
Now the positive part....
The Cayman GTS is a stunning looking car, I too was looking into one only a month or so ago. However, when news of the new GT4 was announced, it put me off, what's the point in having the 2nd best Cayman model? The new GT4 has sold out and is a no go. In addition to that, there's news of a new model/facelift with introduction of a turbo engine. The Cayman GTS has a nice front and side view but its rear end is ugly and too tame. The GT4 sorts that out with the fixed spoiler, looks far better. I figured out I would be ****** off with only 335hp and no way to increase it, Golf R's and S3's would be pushing you out of the way, that's a definite. I would be ****** off with traction and practicality issues too, it's not really an every day car, no 2 seater rwd sports cars are. The Cayman GTS is getting great reviews by Evo/Autocar and the likes but that's based on its track capabilities and handling rather than its grunt. (Let's be honest, 335hp is hardly setting the world alight in 2015)