Rear seats weigh sod all, they are very very light..... The front however weigh a ridiculous amount. I'd say 20kg for both the rear seats and the spare is around right
You also need to remember that losing all this weight at the rear of the car isn't ideal for weight distribution and handling..... You can make the rear end too light and very skittish.
Compensating by relocating heavier items like the battery to the boot area is a good idea when going down this route
Exactly.
I personally think if you're looking into weight saving you've just got to bite the bullet and do a proper job. All interior trim, air con, stereo, speakers, battery relocation, wishbones, wheels, anything steel that can be swapped for alloy, carbon bonnet & wings, lightweight buckets, the lot.
And accept that it'll sometimes be a rather nasty place to sit, but with that lot you'll actually make a proper dent into the overall weight and feel some performance benefit.