The sadev gearbox is 5 kgs more than a standard jc5. 40 kg versus 35 kg.
If a clio cup race car is 900 kg, no way you can get the same shell to 800 under the same fuel and fluid conditions.
As for mk1, evogone, a caged stripped, de-soundproofed mk1 shell is about 900 kgs. To get it lighter than that you have to have lighter windows.
The reason people on here beleive that their cars are dead light is because the official renault williams weight includes fuel. Drain the tank and you knock 50 kg off the weight to start with.
If you knock 50 kg off a williams weight of 990, you get 940, so a stripped caged mk1 at 900 kg is about right.
Someone weighed their williams with 1/4 tank of fuel:
http://www.williamsclio.co.uk/forum/viewtopic.php?t=24598&highlight=weighed
Add in the other 3/4 tank = 37.5 kg = a total weight of 997.5, which is about right with the added weight of door bars in the williams 2, in this case.