I would like to caution everyone who is using this piece of kit to drive conservatively at first and get used to the handling of the car, because that WILL change, probably pretty dramatically when the car is not fully loaded. The reasons are outlined below in simple terms.
The rubber donut that GTO has installed is basically an elastomer (probably polyurethane). Installing this basically makes the rear act as two springs in series, which in theory should actually
reduce the total stiffness at the rear. However, in practice, since the method of installation involves reducing the number of active coils on the coil spring, the stiffness increases quite a bit, which is why the rear sags less when there is heavy load in the car.
From looking at GTO's pics, the fact that the number of active coils on the spring has reduced from 5 to 3 means that the spring stiffness has gone up by about 66% worst case [spring stiffness is inversely proportional to number of active coils, so the percentage change is 100* ( (1/3) - (1/5) ) / (1/5).]
This, guys, is a HUGE change. When a car is tuned before it is released for sale to the public, the front to rear stiffness distribution is very carefully determined because that has a massive impact on the balance of the car. In general, the more the rear stiffness, the less the understeer.
In all probability, this mod reduces the understeer quite heavily when the car is lightly loaded, so you guys who install this need to be VERY careful when driving spiritedly or in slippery conditions cause the rear end can break away in such a manner that a normal (non-expert) driver cannot catch it soon enough. The consequences are left for your imagination.
The correct way to achieve the same effect is to make the springs in parallel, and have one of the springs contact only when the suspension starts compressing heavily under load. This is known as a bumpstop in common parlance, but technically it is called a
jounce bumper, the black rubber part shown inside the coil spring below:
This part accomplishes two things:
1) Does not affect the stiffness when the car is unloaded, thereby not having any effect on handling.
2) Increases the stiffness when the car is heavily loaded so that the suspension does not bottom out and cause snap oversteer due to the stiffness increasing to almost infinity (in theory).
Most of the time this part is installed on the shock shaft, so the Honda probably has it inside the shock, but unfortunately it is not sufficient when the car is heavily loaded, so the customer has to resort to poorly designed but partially effective measures such as GTO has done.
So, in parting, please use caution!!