if you race a LOT, stay with 15s and get some really light ones.
otherwise, i personally think the sweet spot for celica rims is 17"--best combo of tires & rims available.
i think 18"s look better, but they are more to keep up (you have to keep your tranny is excellent condition) and tire prices are outrageous.
could you explain the tranny thing more in detail? coz i have 18's
does that ruin the tranny faster or something?