There is a rule when raising BHP 1/3 of the extra energy goes straight out of the exhaust, 1/3 has to be dealt with by the cooling system and the last 1/3 is your extra power. So a must to have a bigger radiator if you go for significant power gains.
Koyo are pretty well made from what I have seen and fairly well priced, you can pick up a real bargain from USA. However there are a few fake Koyo Radiators going around I read, no proof though.
ARC Rad is the daddy though ^_^