The 90 pounds you removed is from an area of the car that provides your traction. That's why plastic back windows and fiberglass trunk lids are not the best choices unless you have a 4-link or adjustable suspension. Sometimes actually adding rear weight will make a car go faster even though it increases the overall weight of the vehicle.
Hopefully it allows you to run faster, but I know a lot of example where guys slowed down removing rear weight.
Tom