What tires are you driving on and what class are you in now? Before spending any money on the engine, I'd make sure you were running on some of the best rubber available for you. I've always felt when I autocross that I didn't need more power, I needed more traction. There's a limit to power on the tight courses of autocross, and I think you will find little improvement elsewhere. If you want some proof, I'll just give you the results of the raw times of what happened yesterday at my autocross.
Our friend Mike, Qtrflash on here, was running SM in his SVT with his JRSC on Goodyear F1 Eagles, stock suspension. His car definitely has the torque to scoot (way better than the stock engine), and his best time was in the 46.xxxs. I'm running on a stock engine with Kumho MXs on stock suspension, and my best time was a 44.xxx. Without the better handling and rubber, more power is going to cost you your traction.