i need to improve performance

Dear all

Greetings

I need your guidance i am rallying with a 2wd corolla ae101, and i am using a 4age engine blacktop 20v with a 6speed gear box and a A/T ECU.
I made the following modifications thinking that i will get more performance but instead i got a performance decline:
1. i removed the small circular plates which are inserted in the exhaust manifold.
2. i also removed the extreme end back exhaust silencer
The rest is as per original set up, may be it also important to note that originally the car had a silver top before i upgraded to a blacktop.
The maximum i can go when the throttle is placed down to the bottom on a stretch is 100Km/h in speedometer.
Kindly members on this forum advise me on how improve engine performance.