Greddy Mid pipe and K&N CF filter with box
 

I'm sure this, especially the first part, has been discussed before, but I was not able to find a thread answering my question (though I'm sure it was my suboptimal search), but I put on my flame suit before even creating the thread just in case hehe.

I will be getting, practically for free, a Greddy mid pipe and a carbon fiber airbox with a K&N cone filter in it (don't know if the filter is a set or something, could not find anything onilne). I have one question regarding each.

Regarding the mid pipe, I already have the Greddy cat back, my question is more regarding the removal of the cat, and more specifically, what happens to the O2 sensor? Is that removed aswell? If my understanding on how the O2 sensor works is correct, there would not be a point for the 2nd O2 sensor to exist as there it nothing that really happens between the to sesors right? How does this affect mileage though? And also, how is the car able to detect rich or lean mixtures and do adjustments?

Regarding the air filer, I know I'm being very vague about it and will try to get some pics of it to post, but just in case anyone knows which one I'm talking about, is it in any way worth intalling it or is it a bad filter system to begin with?

There should be an O2 bung on the new midpipe. Good luck getting the old one off the cat though. your engine will run fine but you will get an engine light saying that your cat is bad.(of course it is, its not there) i'd recommend getting a new O2 sensor unless you want to try your luck of not stripping the threads.

i didnt know greddy made a midpipe for the 8. are you talking about the k&n typhoon intake? if so v1 or v2?

There are 2 O2 sensors. The one in the cat is related to emissions testing, and will tell your engine that the cat it bad (as nuke pointed out).
The front O2 sensor is located pre-cat, and is responsible for your cars ecu running in open/closed-loop (ie detecting egt's).

If you do yearly emissions testing, it would be a good idea to have an oem cat, with a functioning O2 sensor. Otherwise, you'll just have to live with the CEL :)

Nothing to add on the midpipe discussion as the previous posts cover my thoughts. However, as far as the intake, I would not install it. The K&N set up is pretty bad. A little bit of reading on intakes might help you out decide whether you want to install it or not.