As a rough guide....
Depends where the weight is from relative to the crank centre line, the gearing, the weight removed and the weight of the car
The weight from any internal lightening is approx 1:10 in 1st gear
So 1kg off engine internals is approx 10 kg off the car as you go through each gear the 1:10 reduces
There is a formula in DV's book
It has no affect on top speed, but you will get there quicker....
Megalite Fw, alloy spacers, 3 straps, En24 lightened bolts, Alloy Back Plate Ti bolts. Whole set up 4.8 kg