Does lowering the ride height of a car 50 mm exactly lower the height of C.G by 50 mm?

No. All the wheels, axles, brakes and parts of the suspension would remain in the same position relative to the ground so their contribution to the centre of gravity doesn't change. The overall CoG will decrease by < 50 mm.

Figure 1. For your calculations you can just consider the heights of the two CoGs when calculating the overall CoG.

This is the equation to calculate the height H of CG of a car.

$$ h=frac{ Sigma m_n *h_n}{ Sigma m_n}$$

Or a weighted sum of contributory parts divided by the mass of the car.

In most cars this ratio is approximately 65% super structure and 35% suspension and drivetrain components that are fixed to suspension. So if we lower the cab by 50 mm we lower the overall CG by approximately 35mm.

To make matters more complicated some parts and links are connected to both, such as shocks, springs, steering rods and anti-roll bars.