I'm intrigued as to how you know this, given that the RS3 doesn't even exist yet? And even so, let's assume an S3 with a remap and mods with 330bhp (the figure that people are banding about for the RS3), that'll not do 0-60 in sub 5 seconds.
A TTRS manages the 0-60 in 4.6, but is a fair bit lighter than an S3, so an RS3 with the 5cyl engine will be even heavier, so I can't see the RS3 being as quick as a TTRS, so again probably not sub 5 seconds, or maybe just under if you're lucky.
Having said all this, we don't know what the specs for this car will be, it even ever turns up. But, based on assumptions, I dont think that is going to "rip" a Cayman S, even in a straight line.
Either way a few tenths of seconds either way is not why i'd buy a car anyway. My point was that there is an awful lot to life than an Audi, especially with around £40k to play with.