If there's anything that fills the spot between stock and STI muffler, I think it'll just be the Fujitsubo Authorize S (axle-back) since it's only a few dB more than stock. However, I couldn't find the STI axle-back dB specs anywhere, so not 100% sure.
I feel like any other brand of cat-back or axle-back will be louder. Fujitsubo (an OEM supplier) and STI (also rumored to be made by Fujitsubo) are as OEM+ as you can get IMO.
Part numbers:
- 360-23111 for Zenki
- 360-23132 for Kouki