I would look to the front of the car and focus on brakes and suspension to lose some unsprung mass. I am hoping brakes and bottom arms will save 5kg per corner, then adjust the suspension accordingly and add horsepower. That will be the most effective performance for your money. Battery is interesting though !
CF panels would be very nice but very expensive
I spent a lot of time lightening a semi track car before and it gets very expensive, with doors, wings etc in alloy or Carbon. Usually the big wins are :
Front Seats
rear seats & Trim (If you are prepared to lose rear seats etc)
Doors (may beed a cage to deal with side impact protection)
Bonnet, wings in CF
Wheels, brakes (win, win)
Exhaust (win, win) - suspect nothing much to be gained here
Flywheel (win, win)
Remove stuff you don't need !