Don't forget, the gt4 was 4wd. You can only have so much power going through a front wheel drive car. But I would go Extractors and 2 1/2" exaust. More agressive cams. Change the air filter, Cold air induction (won't help much, but it won't hurt either) See what chips and tuning is avaliable, but the results may be quite small. Some handling mods may be in order, like suspension.
But personally I wouldn't bother, and maybe just put a good stereo in, a Celica would be good for that sort of thing.
I would save the money for a faster car in the next coming years (AE86 , silvia, MR2, some honda coupe, these could be a possible upgrade.).
One of the toyota or celica forums I am sure could help you more, as they will have done this stuff in the past and know what gives the best results.
Greg.