If you're after a new saloon car, but you want it to be fast, you'd probably have to stay away from the premium brands. One car that came to mind was the Mazda 6 MPS, but the new version hasn't been launched yet, so I'm not sure of a 20k saloon that is anywhere near as fast as the Cupra.
For me, hatchbacks and estates are more usable (with mounting bikes etc. - if not quite as fancy), so I'd put the 20k towards a second hand audi s3 sportback or s4 avant.