first off your oem dampers were not designed to be run on lowering springs so you're kind of pushing the limit of the damper capacity. it's too soft. also you're riding on progressive rate sprigns so the initial bump is mush. it would help if you run swaybars so that at least your turn-in response is better.
as for the scraping... that's expected if you lower your ride. this is one reason why im not doing it on my Z because it is already about as low as where my G35C is with prokits and d-specs.
welcome to the world of modding.
hahaha