Pulling significant wieght out of a car isnt all that easy. Once you have done the basic stuff like carpets, interior, spare tyre, tools etc then you need to start getting creative with it. And after that, there wont be many big ticket items. Instead it will be the culmination of all the little small things to hopefully add up to something substantial.
Pulling sound deadening off, gutting doors, boots. Roof and bonnet reo bars etc is one way. Glass to plastic. Removing latch mechanisms and fitting lighter quick release / aero latch type things.
Pulling out the HVAC system (in engine bay AND under dash).
Stripping all the under side body deadener shmoo off. Stripping the seam sealer out of the cracks (and stitch welding the seams while your there.)
Removing the booster, and ABS system and fitting a race pedal system. Remove the hand brake system (including using rear rotors with removable on non-existent drums. My biot rear rotors can remove the hand brake drum totoally from the alloy hat.
Cutting off the front end and build a basic structure.
Can also start replacing the critical components with lighter wieght options. Investing in premium wheels, brake rotors, driveshafts. Remove fuel tank and fit a smaller fuel cell. Lithium battery.
Cutting the spare wheel well out. Cutting additional layers of sheet metal off. (Often there will be sections of the car where multiple layers of pressed sheet metal are spot welded together to make things like the box section where the seats mount or stiffen up-rights pillars which might not be as critical once the car is caged.)
Replacing panels and doors with CF. There is a fair bit of support in the aftermarket for the GD chassis.
Can also either gut and un-pick the OEM wiring loom for a bare bones system, or replace with a new custom made item. There is ALOT of wieght in the OEM looms.
Pull the entire dash out.
Replace no critical bolts with plastic bolts. (Or cable ties

)
Titanium pipework. Cut the exhaust off - side exit - bonnet exit.
Unfortuantly, once you have done the easy stuff, then it gets harder and harder, and more and more expensive, and you get less and less gains.
Could also take pounds off the driver if hes a big boy.