The way i understand the "manufaturers" weight of the car is what they call DRY weight ie 1119kg this means no oil,water,brake fluid, even air in the tyres!
Hence why if you take your car to a weigh bridge its gonna be more than this
I have a striped out car and it does make a difference,but thats also to the road noise and getting in and out with harnesses is a pain not to mention just trying to reach things in the car like the radio or if you drop something!
All things to consider before you take the plunge