Because I am not comfortable with the call in general and adding 500-600 whp to the equation makes it easier for me to admit that I am not ready for that much power in a light car.
What if you set your rev limiter to 4500 or something?
Sometimes I think the issue with a good amount of power is a lack of respect. If you build it with 500-600 hp and drive it with a heathy respect for your lack of experience, you will probably be better off than the guy with 400 hp that thinks he is invulnerable.