I had the exact same question, as I have a tragically high ball flight due to my impact position.
The 6.0, from what everyone else told me, is 1. heavy (120g) and 2. boardy. I was recommended the 5.5 instead.
By going heavier to the 6.0 you may lose some swing speed, but you should definitely see lower flight compared to the lightweight shafts. You won't suddenly hit stingers, but you may see some height decrease. It won't be night and day, at least from my experience.
5.5 is 115g or so, and it could give you the lower flight without feeling too "heavy." However, if you feel good to swing the 120, no problem.