someone on one of the forums has a list of weighed and removed stuff. i remember thinking it is amazing how much you can shift with minimal effort. sure grp/carbon/polycarb is not that cheap, but considering the weight loss it's the best £ for £ mod.
here's some guess work but you get the gist.
air con pump + condenser, brackets, wiring and ancillaries = around 20kg.
I can barely lift a standard door with windows on my own, but a grp door with plastic windows i can just pick up one handed. they weigh >50% less which is easy 30kg saving.
same for the boot and bonnet. so easy another 20kg.
rear seats, interior and sound deadening = 30kg.
spare wheel = 6kg.
tool kit = 2kg.
lightweight battery can save 5kg.
remove the rear bumper reinforcement bar and heavily chop out the front bar = 5kg.
Y bracket and bastard bolts (I'm really scraping the barrel now) = 1kg
rear quarter windows, polycarb vs glass is 50% lighter = 5kg.
1/4 tank of petrol vs full = 35kg saving so worth baring in mind siphoning tank if you are 1/4 mile crazy.
all the above is easy >100kg. for sure the driver is a big part of it but you can see how to get a good 1/4 time with little bhp.
I fully interior stripped a standard car years ago and put harnesses in, didn't change any metal body panels though, but it was so much quicker I was amazed. Shame i didn't get to 1/4mile that car. Horribly loud and clangy, uncomfortable constant road noise on motorway, was fun for 18 months but had to go back to standardish comfy... getting old
lol, plus super rusty arches and take massive toilet break before you hit the strip