To be honest, lightening a car for racing is a much better idea then boosting the power of the car. and I quote Colin Chapman:"blah blah blah...added lightness...blah blah"
The thing is, most people can not afford a second car for the track to race or drift in. They have to build a kind of race car/daily driver hybrid. Loosing weight is good, but it has to be within reason. Lightening your car can also raise your fuel economy. So in my opinion, it is a good idea for everyone to do this.
The back seats of a 2+2 is not for people to sit in, most 2+2 sports cars have back seats that can not fit your backpack. They are just for lowering insurance rates on sports cars. It is just a waste of interior space and extra ballast, So you can remove the back seats without having any remorse afterwards(unless you are planning to sell the car, then you shouldn't remove anything).
The rear window can be replaced with perspex, but I don't know about the legality of this in D1 or Formula D.
The spare tire should be at home in your garage, for emergency, just bring a puncture repair kit or call AAA.
Don't have any junk in your car, don't keep your whole wardrobe or golf bag in the car when you are going to school or work.
Removing the AC on a daily driver is a bad idea, as demonstrated in this trip across Europe with James May in his Aston Martin V8 Vantage N24 stripped out GTR racer.
http://www.youtube.com/watch?v=QApgl...eature=related