Interesting design and looks good. I'd remove the brass weight behind the rear axle and try it in front of the motor. I have used weight behind the rear axle with great success on muscle cars that are high and have heavy body weight. Not sure it would work on a GT3 car though. 21k slotit short can won't be enough motor, go for a NSR 25K shark at least. I think they are a great motor.
95g chassis + 20g body is good for me if everything is balanced and works well. The Slot it tungsten weights to replace the magnets on the pod positions are worth considering as well. I like the brass side pans out wide, Scaleauto deep guides are good and worth playing with the guide depth. Polar length looks good, I'd narrow and harden the front tyres, make them independent with slot it hollow axles. Of course all of this depends on how much traction your rear tyres have.
I could be way off and this setup kicks butt anyway...
- Cam