TL;DR Dont use RTV for header to TP/HFC connection, it WILL leak.
So I got some Resonated Test Pipes from someone on the forum, brought them to my favorite shop (at the time) to have them installed (I didnt want to deal with the demon bolt). The shop calls me a few hours later to ask if I brought gaskets for the header flange, and I said that the RTPs didnt come with them. The guy from the shop said that they would use either RTV or fabricate a gasket, and I told them either one would be fine as long as it doesnt ever leak.
I picked the car up and loved the new louder, deeper sound and extra HP. However, I began to notice that my car had a loud HISS whenever I was decelerating. I rode in a friends Z who had Long Tube Headers, and his had a similar, albeit less loud hiss. It got louder and more frequent until I finally got under the car to see that the shop had used only RTV, and that it was completely blown out. They have only been on for 3 months.
I ordered Z1's metal gaskets on Black Friday (see here:
Z1 Metal HR/VHR Manifold 3 Bolt Gaskets) and finally got the chance to install them today.
Here is the obivous leakage:
So at the end of the day, if you are going to do anything with headers, test pipes, or HFC's, dont be cheap, spend the extra $40 and get gaskets so you only have to do it once.