Keep in mind that not all "acoustical engineers" (or any engineer) are the same. His specialty seems to be inside the room itself, which is VERY important, and the Green Glue is a piece towards soundproofing the room itself.
I've built 4 rooms of my own now (plus part of 2 others for friends/family over the years), and have done all sorts of things along the way, but my latest space incorporates Green Glue and there IS a significant improvement over other products or over nothing between the drywall layers. I would never go back to not using it.
Containing lower bass energy is extremely difficult, and adding Green Glue (or using products like Quiet Rock) by themselves, or even with some other isolation technologies isn't going to do it. Think underground bunker. LOL
As for footsteps, that is called flanking sound and is completely different than airborne sound energy. For flanking sounds, you need to isolate the walls/ceiling/floor from the rest of the house or space that you are using. Decoupling is your friend here.
We have our master bedroom directly above the theater and I can watch a movie at my normal volume (not reference as I still want to be able to hear in 10 years) and as long as it isn't a massive explosion fest movie (think Transformers), she can sleep right through it as the only thing that she can hear is the LFE and bass, but it is tamed down significantly.
I went with staggered stud walls and clips/hat channel ceiling for the isolation, added insulation into the cavities (but only one "side" of the wall and didn't "fill" the ceiling as you still want some air in there), put putty around outlets, created backer boxes for lights, did a layer of 5/8" fresh drywall from a drywall company, not box home improvement place, caulked corners, applied green glue before putting on the 2nd layer of 5/8" drywall with a "double load" amount on the ceiling. Added a solid core exterior door with seals as well.
I made a video of it at one point where I was playing a scene from Oblivion at louder than reference levels, and outside the closed door, yes you could still hear the movie, but when I opened the door, holy crap was it loud. I had a huge grin on my face knowing that I have THAT big of a sound proof improvement without going crazy on cost. I have less than $1000 into all of the "extra" stuff for soundproofing, and keeping in mind that almost $300 was just the door, that really isn't that much.
But putting just Green Glue, using just special drywall, using just insulation in the walls, using just some sort of isolation like staggered studs or clips/channel won't do what people want. It really is a combination of all of this stuff that is needed to really be effective.
(Note: In a previous space, I did double drywall all around and insulation in the wall/ceiling and it was terrible. The staggered studs and clips/channel got rid of the flanking sounds, and the Green Glue really did help with the mids and highs.)