With that front bumper, plus 16" rims, you should be fine with a 0.8" drop. Any further, such as 1.4" or even the popular 2" drop, you could run in to some problems and may even need to totally forget about parking anywhere except on the street.
Consider 1) that while your front bumper may get scraped -- mostly on the bottom, where no one will see it -- the front bumper supports on your fenders could see a bit of flex action, causing them to tear/break off. 2) If you have an extended exhaust, backing out of driveways actually puts the weight of your car on the back suspension, lowering your exhaust even further. With the tip closer to the ground as you back out, there is possibility to scrape your exhaust tip. This will be visible.
As is always suggested though, don't just replace the springs [and never install "cut-springs"]; do it correctly and replace the springs with the appropriate shocks as well. It's good that you are not sticking with stock 15" "GT" wheels. And that you are not jumping right in to the "can a 2in drop work?" mentality.
On a side note: TRD also makes a good shocks+springs kit that works really well for the Celica's handling -- and good looks
