[Bob W.]

I have compared running the 3.3 turbo with single plugs and both plugs firing. There is definately a difference. The engine is a lot smoother with twin plugs-especially under a heavy load. Detonation, of course, is reduced.
However, I also feel (like most of us) that unless there is some compelling reason to twin plug; I don't think it is necessary on a street engine.(Read that as the bucks can be better spent on other things that would enhance the reliability.)
Consider twin plugging if you are going to run high compression or high boost on a turbo engine.
I think installing Raceware hardware, balancing everything, magnafluxing rods and crank, shot peening the rods should be done first. Perhaps shuffle pinning the case, mooning the cylinders and boattailing the main bearing webs could also be done.

Bob W.