First, I'll start off by saying congratulations. I've performed many modifications to my car, but installing SSI's was by far the greatest mod yet. When you start the car for the first time, it is such an instant gratification for your hard work. Just keep that in mind.
When I was getting ready to swap out exhausts, I followed these two links.
Pelican Technical Article: Installing SSI Heat Exchangers on your 911
Pelican Technical Article: 911 Heat Exchanger Retrofit
There were a few challenges I found in installing SSI's.
1. The exhaust stud issue.
I was fortunate. The previous owner had rebuilt the engine right before I bought the car, so the heat exchangers hadn't been installed for very long. As a result, I soaked the nuts each night for a week with PB Blaster and they pretty much came right off. One nut was seized to the stud, so the stud and nut came out together. I just replaced the stud and nut.
If you stay patient and follow the guides as far as heating the nuts, you should be ok.
2. Getting the old oil lines off without stripping the threads on the external oil thermostat.
For this, I knew I was replacing the lines with squeaky clean, new oil lines, so I just got the Dremel out and carefully cut the old lines off.
If your car doesn't have a front oil cooler or trombone oil cooler, you shouldn't have an external thermostat, so this shouldn't be a concern.
3. Unbolting the crossover oil line from the engine.
I had problems getting the old one off. I finally removed the middle oil return line. Luckily, mine are the collapsible type, so I could just reinstall them with new o-rings. If yours aren't the collapsible type, this is probably a good time to replace them with the collapsible ones because they'll be very easy to access when the exhaust is removed.
4. Finally, I found getting all the exhaust nuts fully tightened was challenging.
SSI sends a tool for tightening the nuts that don't have an access hole through the SSI itself. The problem is, Pelican lists a nut that is 1mm bigger than the tool. So be smart, search through the BB and you'll find the proper size nut for the tool. If you mess up like me, get lots of universal joints, extensions, really short sockets, and some creativity to get the nuts tight.
Muffler
When it comes to a muffler, I thought about all kinds of exotic, lightweight options from off the shelf to custom. After doing lots of homework and talking to some local wrenches, I decided to get a Dansk 2in-1out oem replacement muffler. It is quiet when you need it to be quiet, yet when you get on it, it sounds like a 911 is supposed to sound. There are no fitment issues when using them. Plus it's the muffler Porsche engineers intended for the engine.
On a scale of 1 to 10, I'd probably place this at a 5 and that's only because the items are heavy and there are so many things to remove and replace. But it's a project that has been done thousands of times and is very well documented. As long as you keep a reference nearby to follow and diligently keep track of completed steps, you shouldn't have a problem. And if you do end up having a problem, you aren't alone. Somebody most likely made the same mistake, you can search it out here and find the solution.
Luckily for you, SSI's are based on the original equal length exhaust manifolds Porsche intended for the 2.7, so the highest gains possible will be experienced by your engine. The hp jump was definitely noticeable on the top end of my 3.0, but the biggest jump I noticed was the mid-range torque. There used to be a very prominent step in power around 3-3.5k. But now it just pulls, and pulls hard, from idle to redline. All the while making the sweetest music you've ever heard.