I have lightly modified the engine since I bought it. I have added a Pacesetter 4 into 1 header for a Celica GT, but modified it to fit the GTS. I have also added a Vibrant Performance aftermarket muffler, NGK silicone spark plug wires and polished the exhaust ports and put in a K&N drop in filter and drilled holes in the air filter box. For 135hp, and now modified, approximately 150 or so, this is a very quick car. I can beat V6 Ford Probe GTs with it.
My suspension and steering is all stock and the car handles great, but I do think a triangulated strut bar in the front would greatly help under hard cornering.
Seating is 2+2,however as I am 5'10" and must drive with my seat as far back as it goes, that leaves about an inch and a half for leg room in the back. Front seating on the driver's side is very comfortable, and the lumbar support is great. The passenger seat, in my opinion, is very comfortable, but there is not enough side support, as it feels like it is too open, and could use lumbar support as well.
The driver's seat is awesome, I will not trade it for an aftermarket racing seat. The car, due to it's size is hard to get in and out of, as it sits very low.
There are not many aftermarket parts available for it. There are headers, exhuast systems, springs, struts, cam gears etc, but there is a very limited selection. It seems like many more parts are for the lower end model GT, instead of the higher performance GTS.
In conclusion, the 1996-1989 Celica is an ansolutely wonderful car to drive. It is quick, has great handling, is very comfortable, gets good gas mileage (around 550km from a 60L tank running on 94 octane),parts are cheap and for a 16 year old car, it still has style.
I think this car has been one of Toyota's best efforts.