Installed on a 2017 STI. Fitment of this header was very good and it's not touching anything it shouldn't. No prying needed to install, just a little lining-up and I was able to push it onto the heads without a fight. Torque the flanges from the center out per Killer B's instructions and they'll seat good. While you will have that EL sound, these seem to sound a little deeper than other EL headers.
I went from a Tomei EL header to the Killer B header. I was tuned on the Tomei header and, after switching to the Killer B, I noticed a small dip in power before 4K RPMs. I got retuned and the tuner was able to cleanup the power band to make power delivery feel more consistent. Considering the differences between these headers, I don't think it's unreasonable to expect that you will need a retune to take full advantage of them.
About the Tomei EL headers: I wasn't able to get the slip joints loose when I initially installed those headers. I got them installed good enough that they didn't appear to be leaking, but after removing them (~10k miles of use) I could tell that the flanges weren't flat against the heads due to rust on the gaskets that wouldn't have been present if the header flanges were seated flat against the heads.
Tomei VS Killer B: While cost is a big difference, the Killer B header is lighter, easier to install, has a better heat management option (wrap VS Swaintech), and should outlast the Tomei.