Jon, you don't need to buy a lightened flywheel, although specific racing lightweight ones are available - at a premium! Their life span isn't the same as an O.E. part either.
Any decent engine machine shop will be able to lighten your existing one. Looking at the back of the flywheel, material is removed from inside the outer "hub" part of the flywheel, adjacent to the ring gear land.
Obviously you shouldn't remove too much material because it can weaken the structure of the flywheel itself. However, the machine shop should (hopefully!) have plenty of experience in this type of work and know what is a "safe" amount to machine off.
Last time I had one done they only charged me 1 hour labour - £30 or so IIRC?
It won't make any extra power having it done, but it does help the engine rev up quicker improving throttle response and reducing the rotating mass.