Lol as some people have said correctly, faffing with a kg here / there on a 1000kg+ car with 450bhp+ and huge torque is honestly not worth it. Same for most of our pseudo performance cars. Start talking about a bike engined kit car with naff all torque and sub 600kg or a sub 1000kg car with a revvy engine and not much torque (205 gti?!), and yes for the latter a set of poly windows carbon bonnet and tailgate etc start to build a case in truly measurable performance improvements. In the case of my kit car example, have a good dump and trimming your nails before driving can even make a difference! The heavier your car already is through inherent body and drivetrain design (your 4x4 running gear and boat anchor cast iron yb will negate any scraps of weight you can save) the more that power and torque and how the car utilises that is infinitely more important than weight. Stevies video of the gtr is a perfect example of that and actually I'm sure without looking that most of the tin top closed wheel "proper car" super cars which are in the nurburgring top 10 will weigh a lot, but have a lot of power and use it all cleverly. If I was you and i had to pursue a weight saving ambition, I'd be more interested in spending my money on an alloy block than bits and bobs here and there , knock 25kg off in one fell swoop and not stress about the inconsequential. I guarantee you'll go just as fast and not have little bits about the car that would certainly annoy me at least (I want the battery as safe and serviceable as possible... I hated the perspex windows I had in all my cars, they made fuck all difference other than to make life in the car a sweaty pain in the backside... etc).

Your Colin Chapman quotes just aren't relevant - he had to get the most out of small cheap lacklustre engines and became rather good at it what with necessity being the mother of invention etc, by starting with blank paper designs of tiny lightweight shells and running gear (go and stand next to an original elan, scary in size and build). I'm sure he would feel touched by you trying to apply his principles to your car but genuinely reckon as a very clever engineer he'd see there is better returns of modification you could do elsewhere, take you to one side and suggest a change of tack.

Ps the quickest competiton car I've ever been in is pat Dorans rs200... that seemed to do alright in actual proper motorsport, seemed to handle and stop alright (actually the best stopping car I've ever been in!), and you know what? It weighed an absolute fucking ton lol! And that was without pats massive frame in it... didn't see him going on a diet or putting the car on one ; )