The TR280i flywheel weighs 2490 grams. With S3's flywheel weight attached, its mass increases to 3015 grams. I have experimentally determined the moment of inertia (MoI) for both configurations. S3's weight increases flywheel MoI by about 24 percent. The extra flywheel inertia smooths power delivery and has a positive effect on starting. It also diminishes engine vibration as perceived by the rider. Additionally, there is more “in the bank” when you dump the clutch.
I have both the S3 flywheel weight and the one from the OSSA's Special Parts Catalog. Each has advantages and disadvantages. The cover extension for the S3 weight is very thin (only 5mm). This causes the weight to rub against the flywheel cover when the screws are drawn tight. Ironically, the OSSA cover extension is much too thick (10mm). I decided to mismatch the flywheels and cover extensions. Even after I thinned the OSSA extension by more than a millimeter, it is still thicker than necessary.
The S3 flywheel weight has an elegant attachment method that uses “buttons” inserted through holes in the flywheel from the stator side. The OSSA flywheel weight requires drilling and tapping the flywheel itself. I made some S3-style buttons for it instead. But I have been second-guessing myself about that. If the attaching screws ever became loose, it might permit the buttons to chew into the essentially irreplaceable stator assembly. Conversely, if the screws became loose under the OSSA method, the flywheel weight would probably rub against the cover and make a noticeable noise before doing extensive damage. Unless of course, it became so unbalanced as to shear the screws and thereby allow the weight to come off completely. I heard about something similar happening once during dyno testing in which a flywheel came off an ATV and broke through a wall after quite a bit of ricocheting. Scary!