The fly weight kit is essentially just the metal washer that is already on the 255g sun. On the fly it adds weight as they replace the plastic (or carbon) washers. I supose you could double up with two sets of metal washers on the 255g suns (this would add 27g) but you'd need to make sure to tighten the nuts equally or it might alter the balance.
Alternatively I also have the 290 / 275g version in trasparent colours as well (really nice vivid bright colours they are too).
Edit- forgot to mention that the 255g Suns are now reduced in price to £25
