As stated above, the differences between the USDM and JDM versions are mostly suspension-related. People often talk about the horsepower differences and ask, "How does the Japanese 2zz engine make 190 HP and the US 2zz make only 180 HP?" However, that is kind of a misnomer, since the Japanese 2zz-powered Celicas actually make 190
PS, not 190 HP. Since 1 PS = 0.986 HP, then the Japanese Celicas actually make about 187.3 HP (using American standards of horsepower).
Also keep in mind that many dynos of American 2zz engines seem to indicate that Toyota slightly underrated the US-published flywheel horsepower of 180 HP. Stock US Celicas seem to be making slightly more than that. Factor in the slightly higher octane gas sold in Japan (which allows the Japanese ECU to run slightly more aggressive/advanced ignition timing), and you can see that the power difference between US-spec Celicas and Japan-spec Celicas is very minimal.
Here's a helpful HP to PS conversion site to check out:
http://www.flash.net/~lorint/lorin/convert.htm?hpSAE=180&hp_kW=52.2&hpDIN=70.992