Put it this way, I can lift the front end of the shell up off the pallet like a wheel barrow! The heaviest items I noticed were the doors, rear diff, propshaft & gearbox/transfer box. So 2WD would save a huge amount of weight. The rear trailing arms are quite heavy too. The surprisingly heavy items were the Recaros (big frame plus all those motors), the carpet (mint condition sold for just £20 for those who think I'm overpriced!), the pre-tensioning seatbelts and the looms. Even the front radiators are heavy for what they are.
I think a 2WD converted S3 with 1.8T big turbo, fully caged, otherwise stripped, no aircon, with fibreglass doors, bonnet & tailgate, plastic windows, drilled out metalwork blanks, race fuel tank & seat could be done to ~1100Kg with luck. I estimate using the parts I have plus bought new parts (all safety parts, 2WD gearbox, stack dash, new ECU/wiring & respray would cost £10k. Not a cheap track car!