Hey, I have a 94 Z that im about to put a set of jet hot LT's on and was wondering if I didnt put an O2 sim in would it just throw a code and thats it, or would it greatly affect the performance of the car? And I was also wondering how loud this setup would be with the Loudmouth vs a True dual X pipe setup?

 

O2 sims only work on OBD2 cars and only on the O2 after the cat, you will need to relocate and extend your O2 sensor wires.
O2 sims just trick the PCM into thinking that there is a cat and it is working correctly.
You need the O2s before the cat no matter what.
Not that hard to do I extended my own O2s but you can buy O2 extensions from LS1speed.com or other sources.