Reducing "unsrpung" weight is significant in the all around performance of your vehicle. Its one of the highest reaps of benefits for a modified part on the car. Reducing unsprung weight at the wheels has to do with less weight to carry around and handle. Area that are benefited by reduced unsprung weight, such as a 4lb reduction at the wheel.
Benefits:
1) Improved braking performance. The brakes can slow down something that weighs less, its simple, and this is rotational weight too, so reducing that is a huge plus in stopping fast. With less weight at the brakes to stop means less heat, so your brakes will work at their peak performance for a longer duration.
2) Acceleration and range - As with braking, moving or starting this weight in a motion takes energy and power. So range and acceleration are improved.
3) Suspension - As with braking and acceleration, your shocks and suspension components have to work with this weight. It has to slow or dampen the energy so it is not transmitted to the vehicles chassis / suspension components which degrades performance as the suspension needs to work harder. With that rebounding of the shocks is improved which means your wheel stays in contact with the pavement longer, which then translates to more traction and better handling of the vehicle.
4) Comfort (this is actually part of 3 above, however since the Roadster has such a rough unforgiving ride over bumps it really should have its on bullet point) - Since the suspension is working as it almost should (You actually need to install Monoball / spiracle a-arm bushings to have it [suspension] work 100% right due to drag and binding of the stock bushings and resistance), the car has a softer ride and can handle the bumps better for comfort. But as I mentioned, you need to upgrade the bushings on the A-Arms to reap all the benefits fully here.
The upgrade of the two piece rotors is a function as well as a well needed safety upgrade for the Roadster. The stock pads are horrible with the stock rotors, the CarboTech / Gloc brake pads offer a significant improvement to the Roadster's braking, however the rotors are a very weak link in achieving optimal and what is actually required braking performance of the car. One of the many issues and safety concerns of the stock rotors is that right after you take the Roadster for a drive from the rotors being wet, this is right after the rotors have coated themselves with a layer of rust from the rain. (Washing the car or a nights worth of rain hitting the car while letting it sit is all it needs to rust, even a drop of water will make it rust. ) You then find out that the car can not and will not stop as you come up to your first stop. The upgraded rotors help prevent that from happening since there is a two part metal treatment process they're subject to. Also the slotted design acts as a scoop once the brake pad has wiped the facing surface. Then next pass of the following slot has a cleaner area to bite since dust, grit, as well as gases (when heated) have a pocket to go into and cleanly blow out. The stock drilled holes in the stock rotors collect brake dust and dirt. They don't clear out and when you do clean them, they fill up instantly. So they're not efficient at all but rather contribute to poor braking performance. Drilled rotors are known to crack more often than a regular or slotted rotor when put to extreme stress.