Because the Buell uses a pull shock aftermarket options are limited and expensive. The Penske is proported to be the best, but it'll set you back a grand or more.
The stock Showa works reasonably well and when in good condition has a bit of adjustability, but they are prone to leakage and are not easily rebuilt. As the numbers of tubers out there has declined the number of suspension shops willing to fix the showas has also dwindled.
Race Tech is still fixing them, and can improve them to a degree with their own shim stack "gold valve". A simple re-seal and recharge will restore stock performance for about $250. The addition of the gold valve will up the price but it does make the shock work better- this is the route I went and I have been very pleased with the results. I think it ended up costing me close to $500 once shipping to and from was factored in, still considerably less than the next best option.