Sorry for the delay in response, we have been going crazy in the shop!
1. There are no known fitment issues regarding the downpipe with the STi Tranny, or catback.
2. We do include an O2 bung AFTER the high flow cat rated for emissions testing up to 3.8L, in-house testing shows that a CEL will not be thrown but we can not guarantee that based on the exact state of your cars tune. ie. if you are using engine management software and are running rich you may throw a CEL.
3. As with all of our exhaust components, if your current hardware does or is supposed to mate up to the stock downpipe or catback, then our units are guaranteed to fit with no problems or leakage issues. We obviously can not control other product fitments, I can tell you ours fit perfectly to stock or to each other!
4. Hey Dave, looking forward to getting you back in to install this puppy!
Hope this helps to answer some of your questions. Please do not hesitate to ask us questions or simply would like additional info, we are here to help!