I have about the same setup on my 2000 v6. The next thing I am looking at is rocker arms instead of the cam. Engine wise you cant do much more without upgrading your fuel system. But gears will help, not too expensive for the difference they will make.
It has no performance gain at all. It's restricting the air flow to your engine if anything. If you think about it, it has no benefit. It sits in your intake, and spins. It supposedly acts as a turbo with the impeller "forcing" air into the engine. Well, if there's no outside source spinning the impeller, and its only means of spinning comes from the incoming air flow, how can it produce more force than is required to spin it? Example: blow on your hand. Now stick one of those air freshener propellers in front of your hand, and blow on it. You have now seen the spiralmax in action. It spins at the exact rate of flow of the incoming air.
ya know, I think when one of these threads opens up about a tornado or spiralmax, they should immediately be locked just after a server op posts a big fat no.