19's will slow you down a considerable amount. The excess weight, as well as the excess mass of the wheels, will be noticeable. I went from 15" to 17" on my last car and could tell a clear difference.
Also, you're going to have problems keeping the wheels in good shape. Any sort of pothole and you're likely to chip or even bend the wheel.
Largest I'd go on a Celica is 17"... but that's just me. 18 would be okay if you really want the larger look.
Whatever acceleration you loose by mounting 19's, you'll easily regain by some simple modifications like an exhaust. That's how small the difference is. And I've tried 16's+stock exhaust, then 19's+big/high-flow exhaust, then back again to 16's+stock exhaust. That's with the celica.
Any pothole big enough to bend/chip your 19's will also bend/chip your 18's. And we're talking a BIG pothole + dumb ass driver to manage that. You got to take some care driving around with big wheels, but you don't have to be hysterical about it.
What some people do is they check what tyre pressure the standard 16 inch wheels are supposed to have. Let's say that's 36 (I won't bother to check if that's the correct amount right now - might be 34 or 38...) anyways... Dumb ass'es see that 36 is the correct tyre pressure for rims on the celica. Then they get 18's or 19's and they still have 36. And, ofcourse, their rims won't last a month. What a lot of people miss out on is the fact that 35 profile tyres should have 42 in pressure, and 40 profile tyres 40-42 in pressure.
I had 8,5x18's with 215/40 for 3 years. The car was my daily driver. No problems. I had 16's with 40 profile tyres for a year. Daily driver. No problems. I've had 2 different sets of 8,5x19's with 235/35 tyres for 3 years. Also my daily drives. NO PROBLEMS. And the road standards in norway is *shit*.
You have bumps and cracks in the roads so bad your change jumps out of your ashtray, and your cdplayer skips. Still, all my rims have survived.
Get 19's now. The celica's made for a big phat drop and 19's!