I have the invidia and kinetix hfc and them together is a good combo. in terms of mean/loud my favorite has always been ark grip cat back and berk high flow cats
edit; oh, so regarding the gemini... it is a little louder and deeper than stock and sounds good. The HFC definitely amplify it a little more so thats nice, but it isnt enough to wake the neighbors. It gives the car a nice little meanness to it but doesnt attract all the attention in the world.
here is a clip on the dyno -
https://www.youtube.com/watch?v=ThIGjn2hCKo
I had a rattle and was trying to bring it out so I could troubleshoot, but here are 2k - 5k revs
https://www.youtube.com/watch?v=4_0pd7M9_Ww
another accurate sound clip from a good microphone (skip to :40) [
https://www.youtube.com/watch?v=jcNOwau_xh4