Just for information purposes this I/C is not from an S3 but it is an universal that will fit to alot of cars.
This I/C will fit your cupra no probs and will be sufficient until 260-280bhp. I find the Forge one to be way too overpriced. With 600 pounds you get an AMS beast that if you see the build quality from close you are gonna crap your pants and is sufficient for cooling 500wild horses. In order not to be misunderstood i am saying the Forge one is good but just to expensive.
Regarding your other question I say together with the Induction Kit you ll be defo sorted especialy since you live in UK that the weather is pretty cold all year long, because i use a similar size I/C i can tell you that depending your cruising speed you are looking at 2-7Degress Celcius hotter temperature than the environment, max i ve ever seen was like 15 degrees higher after some rinsing.
Because i noticed that you are looking for something relative cheap bare in mind that apart from the I/C you will need silicon hoses(get those with 4 or 5 layers) also, that will roughly cost you 80 quids, dont go el'cheapo on the hoses not worth imho.
Edit: it is not gonna lower your Engine Temperature but the intake Temperature wihich means the air has more density when it colder hence making your car working healthier and going faster since Turbos need as much cold as possible.
This is a modified car forum! I'd go for the Forge Twintercooler since its a tried and tested intercooler modification for the mk5.
