Instead of more oomph out of your car, I would suggest a thorough revision / repair / maintenance of the suspension, brakes and the drivetrain (diffs, clutch etc). Go through the whole engine changing oil, plugs, filters, verify all the hoses are good, fresh coolant etc. Go to a rolling road and benchmark your basic power figures. If down on power, check for exhaust blockages, verify that the boost is at the original level (no need to raise it), and that the air / fuel mixtures are to spec. Stay within factory specifications! The engineers have spent millions to perfect the vehicle, no need to skew the balance. Oh yes, and change the tires to original spec if they've been changed to a different size and / or brand.
I know for a fact that the skill required to properly drive a 200 BHP+ vehicle with AWD and rally-developed chassis will be far superior than that possessed by 99% of the drivers in the world. You will be amazed at the pace that this car can carry around corners if set up to original factory specs. Do some spirited driving around some curved roads and you'll start to understand why the Celica is so special. Enjoy it while you have it! 