The combined approach makes sense — adding a sound-deadening layer under the factory insulation seems like it would address both vibration and airborne noise. I'm curious whether the peel-and-stick material alone made a noticeable difference before you added the factory piece back, or if the improvement only showed up with both layers in place. On the 84-86 question, I haven't seen one with it personally, so I can't say how common it was. Worth checking whether the hood bracing on those years has the mounting points for the insulation clips.