I think any high flow cat you go with will give a p0420 error code without the ECU tune. I might be wrong on that, but that's what I've come to understand.
I went with grade 8 bolts right when I put it on. Don't get me wrong, the bolts they send are okay. But I am just paranoid.
You might want to consider a Remflex or graphite type gasket to fix your leaks. I have had very good luck with them, and it might help you out as well.
I wasn't knocking the Werks high flow cat at all. I've been very impressed with the performance gain, and I couldn't imagine going back to stock. I think the noise is more due to the fact that more air is being allowed to go through the cat than was allowed with the stock setup.
Nighttamer had the same issue with his Clear Image high flow cat.
I don't regret my purchase at all.
