I installed an oem hoodpad recently and noticed quite a difference in the noise level, not so much inside the car but def outside. And with rubber cement, the hood pad is nice and secure. I think adding a butyl layer would be unnecessary.
If you have the carpet out def use the butyl where you can.

__________________
DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School
www.Newgateschool.org - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV