I just installed the bert street cat back. No other mods whatsoever. I can't say I'm 100% happy. Maybe I was expecting too much, but I wanted something that was more raspy and that would really wail near redline. The berk street is OK. Makes plenty of noise, but it is more boomy than raspy and it actually appears to be less loud as I go through the RPMs. Kind of pewters out on the top end. Then again the whole engine pewters out on the top end.... my last car was a 1990 Ferrari with Tubi and rev limiter moved to 8K, so I have an entirely unreasonable comparison in my mind
Having said all of that, the berk street does not drone and is loud enough. I certainly would not want louder for my 135 since it is a daily driver. And it is much better than stock with the golf tee mod. I think the core issue I have is that these turbo engines can't replicate the sound of a V8 or V10 in the M3s or M5s which is what I'd love to have in terms of sound.
Hope this helps