I use that intercooler on my stage 2+ K04 Cupra, you can feel a slight decrease in power in hot summer days, here in Bucharest we get over 35 degrees Celsius. But this is rare, and in UK you'll be fine.
For stage 2-2+ you need 4 things, S3 intercooler, LOBA/APR/Autotech HPFP, Exhaust, cold air intake. But then you will feel the need for bigger brakes (Cupra/S3 with a good pad to start), and better suspension.
Cheers!