My Super Six was originally built with a stock 4 tail. As I began to race, and started to do speed mods that increased the top speed, I found myself cornered...actually squeezed between, Dave Anders and John Huft (modifiers and racers of considerable note) during dinner at AirVenture Cup. They said in chorus, "when ya gonna put a counterbalanced rudder on that beast?" So I built a slightly modified and strengthened 8 V-Stab and Rudder, and it has flow well in this configuration. The mods included pinning the stiffeners together to make them more similar to ribs, adding a doubler to the aft V-stab spar (to absorb the inputs from the stiffer rudder and avoid just moving a potential point of failure), adding a small, calculated amount to counterbalance weight, and doubling the connection between the counterbalance cavity and the main structure of the rudder. All done with some consultation with an appropriately knowledgeable aircraft engineer. The added height did cost a bit of speed (2-3 knots).
I've flown HRIIs with a 4 rudder, and they all flew great...none seemed to exhibit any tail-wag. Elevator authority was always good too, but when 2 up, the bigger bloke has always been in the front (I was instructing the owner then).
An 8 tail works well, but as Mark said, flown conservatively in descents, the 4 tail may be just fine. If you want to go race-fast, or are concerned about overspeeding in descents, the ultimate win-win here may be a modified 4 tail, built with a counterbalance. Greg Nelson built and raced a beautiful F1 Rocket with such a tail, and the tail is a work of art...the plane even has 3-axis electric trim. The plane is very fast as well. I believe Dave Anders and Robby Attaway built similar tails for there 4 and 6, respectively. Were I to do it over again, I would go that route. You may look into those gents' accomplishments in this area.
Another mod that Steve Smith and I have talked about is using pressed-fit bearings instead of rod-end bearings for the elevator and rudder connections, where possible. Steve may be able to add some more data points on this as well.
Cheers,
Bob