the medium really doesnt effect growth speed really, its the way your groign the pant, i.e w hat method of hydro? dwc, aeroponics ect.

as for staying is a rockwool cu bes yea you can but i dont suggest it, simple reason is y ou dont want light hitting the rockwool or going past the rockwool into the res. H ydrotron works great and is really cheap, it keeps the cube coverd from lights penitration, and also su pports teh platn when the roots gro throughout it