I know that a turbo kit is coming out, availible for GT-S automatic. It will cost about $3650, plus shipping and installation. Celicas already have a high compression engine, so the HP increase will not be as much as it is with other cars.
There are other options-they icrease HP much less but are also significantly less expensive (less than $1000). You might try a cold air intake system, supercharger, or NOS.
Manuals are faster....but only in the right hands. I've heard lots of horror stories about people who don't know how to drive them ruining their transmission.