As JB1 says (and I forgot to mention) ventilation makes a huge difference. I don't have ducting but I have cut large holes in the insides of my front wheelarches and leave the fog light covers off (Blob STi). Gets air behind the wheels and does the trick. Also put similar pads to rear brakes to keep braking effort even and not overload the fronts.
OE Brembo discs weigh 10kg, Discs on bells PF for Brembo or AP are both about 8kg so either a useful saving. Also decent lightweight wheels PFF7 ot TD 1.2 are 8kg vs OE STi golds at 12kg. Difference is noticeable.
Strip interior of an STi and fit lightweight seats saves c70kg, sadly roll cage adds 28kg but stiffens chassis.