lighter flywheels cut out rotational mass, they dont add horsepower themselves but they make getting into your power zone quicker. As to how light you should go, you nailed it, its all preference. I would say that unless you want to be rebuying everything when you go boost, to stay away from the really light flywheel like the 7lb one, there are agruments both ways, but turbos dont play well with lightend flywheels, atleast not too light. I wouldnt go much lower then 12lbs