I'm making 300wkw from the stock evo 7 cooler, and 350awkw through the Stagea auto on a cut down Jap core 600x220x76. Obviously you can get better results the larger you go, especially if the cooler pipes are large diameter, but there is a price to pay, and transient response will suffer. Peak KW is only part of the story.
If you plan to lean on the car hard, such as dyno and track work, go for the largest and thickest cooler you can fit without cutting bits of the car off. Otherwise you would be better off keeping what you have for the road most likely, best bang for buck I say...