It honestly isn't doing it for me. Being PE, it'll cost twice as much as it's QC is worth. And I also am really not digging the overall design as Optimus. (The shoulder wheels, tiny waist and torso being a giant square block)
Not having a Roadburn, or wanting the head, I am pleased to see that the pistol is available separately. A tempting purchase for Rust-Dust or someone who could do with a solid side-arm. (Actually - would do Strongarm well in place of the carbine she comes with. That feels too "SWAT" for a rookie beat-cop somehow).
Yes, it's a replacement part for the bits that become the hip panels. It alters the transformation slightly so they stay on his waist instead of hanging down the side. A much cleaner look. I am very tempted by these, but want to see a video review to be sure it all works.
The original has hip panels and a big hole in the side of his waist: