Tridge,
I use one on a IO360 w/ electronic ignition and AFP injection. The prop is the Whirlwind 151. If you use a light composite prop such as the 151 (29 lbs), the extra inertia comes in handy at idle speeds. The prop is very smooth. I purchased the aircraft and have never flown without it, so I am not sure if there is a big difference without it.
The 11 lbs on the nose could be an issue depending on how your aircraft is set up. The combination of extensive electronics behind my aft baggage compartment and light prop on the nose affords me plenty of latitude on my center of gravity. In fact, the builder had to put both batteries up front to get a better balance.
The dampener appears to be a quality product. The company that actually builds them is in California and specializes in building dampeners for top fuel dragsters. Everyone I have talked with, including Vans, say they make a difference. As long as your CG can take it, I dont see how it could do any harm.