Crzy8man

Just going to start out by saying love this site tons of useful information! This my first post but here is what I am needing some help with. I bought a 2008 G37S with about 131k miles on it in September. My dad test drove it and said it drove fine and everything. But after buying it as he was driving It off the lot a CEL came on and it ended up needing new Catalytic Converters. The only place near me that would install them was a dealer and I went with ISR Performance HFC after seeing how much they were going to charge me. Since I have gotten my car back I have not been impressed with their work. They stripped a mounting thread in my K&N intake and didn’t tell me tried to blame it on me since Im in college. This past weekend though I was hanging out with my friend and he noticed my car sounded louder than usually (he heard it right after the HFC’s were installed) and he thought it was a exhaust leak. I have been reading up on this and I think I do have a exhaust leak before the HFC. Here are my thoughts about what could be it and if anyone else has any other ideas.
1.) Some other info my car is throwing the Sensor 1 bank 1 code, and also has a valve cover gasket that is seeping oil. Could this be valve cover gasket be considered a exhaust leak? I have read that code can be caused by a leak.
2.) Is it possible that the bolts holding the cats to the header have simply come loose a little or wasn’t tightened enough?
3.) could that gasket have been put on wrong?
4.) maybe the gasket wasn’t well made and needs to be replace? (if so what’s a good replacement I have seen Z1 and Berkley are both well liked)
5.) this past Friday I also scrapped going over a speed bump. but when I looked I didn’t see anything badly damaged.
I am in college so the only time I’m able to get under the car and look for anything is on the weekends. I looked under my car this weekend and didn’t notice anything major. Thanks in advance for everybody’s help. And sorry for this for being kind of long lol