When it comes to the total performance package engine work will give you the least gain for the money spent. You can improve the performance of the car by working on the suspension and braking systems for a lot less money.
Engines simply aren't a good return on investment. Just consider who much time your spend accelerating on the track compared to how much time you spend going around curves and braking.
The other thing you need to remember is that a high horsepower high rpm motor is really tricky to drive. Power is all about torque. I've worked with a few ALMS teams and they focus entirely on torque. Some of the new engines reach maximum torque at about 3,000 rpm and the torque curve stays flat all the way to 6,000. They exit the corners almost like they're shot out of a cannon.
Focus on the braking and suspension systems if you want to be fast. Focus on the engine if you want to run up your Visa purchases.
Richard Newton
Autocross Performance Handbook
Wheel and Tire Performance Handbook