(why should a curved duct + blocker be less efficient than a full-LOS-blocked duct?)
Don't know exactly why (or if) they must be, but the Super Hornet and F-15 Silent Eagle with RF duct blockers have about the same forward RCS as the T-50, all of which are significantly less stealthy than the F-22 and F-35 with the ram lined S-ducts. So something is going on there.