However fast the TT-RS may be, it's a front wheel drive overweight coupe based on a Mk5 Golf chassis.
The Cayman S is a perfectly balanced mid-engined sports car, designed as..... a mid-engined sports car. It's not about 0-60 times, it's about how well it can maintain the speed around the twisties, and how much fun you can have doing that.
Hmmm, but that simply isn't true now is it?
TT-RS is not on the same chassis as a MK5 Golf or an 8P Audi.
A significant proportion of the chassis and shell are made of aluminium, specifically in the front end in order to imporve weight distribution.. the battery is even moved to the boot to help this.
Have you driven a Haldex Gen 4 car hard? The Gen 4 is a monumental leap up from the Gen 2 Haldex, in standard trim it behaves better than a Gen 2 with the haldex controller system set to 'Race'.
The TT was designed to be an every day sporty coupe. I'll not try and imply it has the feel of a Porsche, it hasn't, and that's why a similarly specced Cayman S would cost you about £55-60k as a TT-RS specced to £50k.
Have you driven a TT-RS though? Its more than capable in teh twisties, and capable of getting a little bit of cornering attitude, so it is fun.
As for holding it's own in the twisty stuff:
Evo lap times:
Boxster S - 1:28.1 peak speed of 105.4
TT-RS - 1:27.1 peak speed of 108.1