to get the best from standard brake setup, you should use group n discs which are made out of a softer steel, so heat up faster and have a better CoF. Used with Ferodo DS2000 or DS2500 pads, it will be the bast you can get from teh standard set up....this will cost around 100 quid, and its what i use, no point in goin larger as i can still lock wheels easily.
The you can get larger discs that use standard calipers and again, use the ferodo pads....can get up to 280-300mm discs....about 150-200 quid inc all brackets.
last is the full monty. Cheapest are willwood 4 pot dynalites with 280mm discs which go for around 850 quid. The the tarox 4 pot at 1000 quid then tarox 6 pots for about 1300-1500 quid and max out at 1800 quid for the brembo 4 pots. I would recommend the willwood 4 pots as they are really all you need for such a light car, and at the cheapest is the best option....as far as new calipers go.
choice is yours!