What exactly does changing the weight rollers do for the scooter performance wise?
Having the right weight rollers will affect the smoothness of acceleration through the range. Choosing the right rollers will depend on your body weight, the altitude where you are riding and any other mods that you've done to your scoot. It's a case of experimenting until everything is just right. If acceleration bogs around 30mph, that's a sure sign that you need to adjust the roller weight.
Also, the rollers wear over time and develop flat spots. This will affect your performance more and more. So, it's always a good idea to check them fairly regularly.