I fitted a lightened flywheel to my 748 last year.
The engine feels more responsive and does spin up quicker. The other "benefit" is that the engine braking improves to due to the lightened 'wheel allowing the engine to slow down quicker.
The only disadvantage can be lumpy/poor idling. Generally though with most of the wheels I've seen this wouldn't happen as you have to fit a really lightweight item to suffer this.
They generally go for about the £120 mark. Fitting takes a couple of hours.
IMO worth doing
