i think its
Then you obviously don'tknow much about cars, shift kits have been around for decades, they recalibrate the valve body in auto transmissions, lessening the time between gears and increasing fluid pressure in the valve body itself. these make a big difference, not as much as a decent torque convertor, but nonetheless, they are quite effective.
TransGo has been around for a while, but many of their kits aren't quite performance-oriented shift kits for the most part. most of their kits are used in rebuilding a worn auto tranny. Make sure if you order one from them, that it's a performance shift kit (street/strip kits usually give the best compromise for racing and streetability)
Auto's like that on the GT are not meant to be manually shifted via the gear selector, rather, they use a transmission kickdown cable to downshift the car when you press on the gas. Some kits are designed to allow you to manually select gears with the shift lever, and shift on the fly without damaging the transmission (sort of like upgrading your tranny to a tiptronic) only it doesn't rely on an electric solenoid to do this.
These are not all that easy to install yourself anymore, it's best if you get a tranny shop to do it for you.
Level 10 is another source for these. I also think that their pricing is a little bit understated; $300 installed, maybe in a Turbo Hydromatic 350, but on a modern tranny like those in the celica, look to pay closer to $500-$600 for the same job.
Also, TurboEast (used to be TBKO) has just re-issued the ESK (Electronic Shift Kit) for the Lexus IS300 and other lexus models. These will work on the celica (due to similarities between the IS300 and celica GTS trannies) although you must splice it into the harness (2 ECU connections, two sensor connections on the tranny, power and ground) They were going to make an application-specific version for the celica a while back, and I have e-mailed them regarding this, still no word back yet. their site is at http://www.turboeast.com/
These work by controlling the solenoid in the transmission, and maxing out the fluid pressure. it is almost as effective as a traditional shift kit, but it can be switched on and off to suit your needs. downshifts with this unit tend to be a bit harsh, but this can be fixed by putting a WOT switch on the unit, so it only engages at wide open throttle.
Shift kits firm up your shifts and make the ride less cushy in the process, but they also increase the longevity of your transmission (firmer and faster shifts lessen the wear caused by the increased slippage in the factory configuration)