I wouldn't really worry about the radiator at that level of power.
Turbo is by far the best bang-for-your-buck power up. All depends on what you want. You can't beat the sound of an NA car etc. There's just something about NA that is really nice and unique I agree, but you will pay a lot of money for any substanial kind of power gain, and it will never produce the same torque as a turbocharged engine.
But for a nice NA setup, I would go:
- datrats or JUN lightweight flywheel
- tomei cams
- fully programmable ECU (raise revs to 8000rpm)
- SR20 throttle body with adaptor plate (quads ideal, but hard to get outside of Japan)
- really efficient air intake
- small higher compression head gasket
- 2" press bent exhaust, 4-1 extractors for high revving power or 4-2-1 for more midrange drivability on the street.
- strip heaps of weight out of it
And the ultimate is to buy some extra japanese parts like: aspect or mulholland high compression pistons, and mulholland 10,000rpm crank!
I would hate to think of the cost.!