Cusco limited slip differentials for professional motorsport use. These are definitely not the cheapest, but are top quality products which are also fully serviceable.
Turbosmart's model specific dump valves are namely a bolt-on blow-off valve upgrade which is both hassle free and designed specifically for each car to deliver the best performance.
Air suspension kits offer a comfy ride with heigh adjustment available at any time. Air-kits are road legal, model specific and are bolt-on installable.