How significant is the effect of replacing with lighter-weight wheels?

The essence of replacing the wheel hub is to reduce the sprung mass (beauty is secondary).

What is the sprung mass?
The blue lines represent the body part, the red ones are the wheels, and the other series of components. The connection between them is the yellow lines representing the springs and the green part representing the shock absorbers. Of course, there are also a series of suspension mechanisms on the actual car to constrain the movement of the wheels. This part is omitted here due to my inexperience.
Structurally, the sprung mass part includes the red, yellow, and green parts shown in the picture, namely (but not limited to): the wheel hub, the tires (including screws), the tie rods, the springs, the shock absorbers, the tie rods, the braking assembly (brake drum or brake disc + bellows), and for some models, the transmission shaft, the overall bridge, etc.
In fact, you can think of it this way: the reason why the picture is drawn in such a strange way is to make people who have no concept of the sprung and unsprung masses imagine the car as such: the car can be divided into two parts, one part is the part that rolls forward against the ground, and after removing the springs and shock absorbers, it can still be placed on the ground steadily. This part is the unsprung mass. The other part is the upper structure where the passengers sit, and if the suspension is removed, this part will collapse and kiss the ground. It must have springs and shock absorbers to support this part. And this part is the sprung part. The two parts are connected by springs and shock absorbers. (Of course, springs and shock absorbers are generally considered as the unsprung mass.)

Understanding this is to grasp the fundamentals of this problem. This is also the reason why I am not satisfied with the explanations of the optimization of the sprung mass found in the current automotive media, which is to build this simplified model and then think about the optimization of the sprung mass. This will be much clearer after considering it.

The significance of reducing the sprung mass
There is such a saying: 1 kilogram of sprung mass, 10 kilograms of unsprung mass. This means that reducing the sprung mass by 1 kg will achieve an optimization effect approximately equivalent to cutting 10 kg from the unsprung mass. This statement is, of course, only circulated in the folk, and the actual situation is more complex. It needs to be considered from two aspects: separately considering the benefits brought by reducing the sprung mass, and comprehensively considering the impact of the sprung and unsprung masses on the car.
First, let's look at what kind of impact reducing the sprung mass will have.
It mainly manifests in the acceleration and deceleration performance.
This is what is meant by 1 kilogram of sprung mass and 10 kilograms of unsprung mass.

The principle is easy to understand. As a component directly connected to the axle, the rotational inertia of the wheel hub and tires has a very direct impact on performance. Whether it's 250 PS or 280 PS, 350 Nm or 420 Nm, you must first overcome the rotational inertia of the wheel hub and tires before the torque can be transmitted through the wheel hub and tires to the ground. Reducing the weight of the wheel hub and tires (including the rotating brake disc) can make the power transmission more direct.

However, just considering the impact of reducing the rotational inertia on the acceleration and deceleration performance, there is nothing to do with the bellows and tie rods. Because they do not rotate with the wheels, their hindering ability is no different from the unsprung part. Then, why do those who like to change the bellows and discs and like to weigh them think this way?
This involves another aspect of consideration.
The ratio of sprung to unsprung mass
This requires the use of the simplified model handily created earlier.
For a car, the road surface is definitely not as smooth as a mirror. Not to mention various potholes, speed bumps, manhole covers, and various stones will cause vibration, and if you carefully observe the asphalt road, the surface is quite rough.
But in a comfortable car, we feel it as silky smooth. In addition to using a softer spring and shock absorber combination, increasing the ratio of sprung to unsprung mass is also an effective means.
Trucks have such an interesting characteristic： when empty, the vehicle shakes terribly, and it has to load something to drive properly.

The reason for this is that the ratio of sprung to unsprung mass has increased.

Let's go back to the simplified model. All the bouncing on the road is initially applied to the unsprung part. The bouncing of the unsprung part then needs to affect the sprung part through the springs and shock absorbers. In a stationary state, the weight borne by the springs is the weight of the sprung part. When the unsprung part experiences bouncing, if the springs compress or expand, it will disrupt the balance and generate additional pressure. According to Newton's second law, the pressure applied by the springs to the unsprung part will also be applied equally to the sprung part. At this time, there are two options (or both can be adopted):

First, increase the mass of the sprung part, just like loading a truck with goods, by increasing the mass of the sprung part to weaken the acceleration caused by the bouncing of the unsprung part transmitted through the springs. In a nutshell, it is by the weight of the vehicle to hold the bouncing down.
Second, reduce the mass of the unsprung part, so that the rebound force required for the unsprung part to produce the same bouncing is reduced, to reduce the impact on the sprung part.

In summary, the ratio of the mass of the sprung and unsprung parts needs to be increased.
Of course, the ideal situation is that the mass of the sprung and unsprung parts is both reduced, but the reduction in the unsprung part is greater than that in the sprung part. The overall ratio of the mass of the sprung and unsprung parts still increases.
Therefore, some people often say that a heavier vehicle drives more stably at high speeds. This is somewhat reasonable, but a more accurate statement is that the ratio of the mass of the sprung and unsprung parts is large, and it drives more stably.

The indispensable springs and shock absorbers
As the part that connects the two sections and is responsible for supporting, transmitting force, and absorbing impact, the choice of springs and shock absorbers has a greater impact. How much the force of the bouncing of the unsprung part is reflected to the sprung part is more determined by the springs and shock absorbers.
So: Modification is a systematic engineering process, and it is essential to avoid one-sided modification.