with typical mods (mods other than replacing a tranny or raising your redline) the more power you add the faster top end you will have.
its important to note, though, that adding gobs of power is useless if you want to increase top end
and have a stock tranny and typical redline. your cars tranny can only move the tires so fast before it breaks (too much redlining). your stock celica is made to get as close to redline as possible in all gears. to make a car that wasnt close to redlining in ALL of its gears is inefficient--and just isn't done by manufacturers because of this.
adding power is usefull for going fast faster
not really for the reason of top speed.