then you are totally fine. Direct bolt up. Get new gaskets and new hardware. The copper nuts are handy, if you need to pop the muffler off later. And copper antiseize. The old muffler bolts will be trash, unless they put new ones on when they put the SSI on.
Backpressure seems to be a bit of a black art. Most will tell you Porsche knew what they were doing, but I don't buy into that (in regard to our midyears) because ultimately they had to account for emissions which threw a giant spanner in the works. In my personal experience free flowing muffler is more power. That's why you see straight pass through on rally cars. Search some of Bill V's posts. I think he has a graph showing various mufflers/straight pipes on engines and performance gains.