If you look at a stock and I mean stock (30+ year old stock) 1.7 muffler and 2.0 muffler you will see the 2.0 is just larger, hence the need to bring it down a little more or make the pipes straight. The 1.7 muffler is smaller so Porsche was able to bring it up a little farther to better hide it. You can use the 1.7 HE on a 2.0, I would doubt you would actually notice a power loss. Both engines are under 100hp and differ by less than 20hp so the difference would probably be 1-2hp loss. Somewere in there.
Yes, the SSI's are slightly larger than even the stock 2.0 HE pipes. Now, I set of SSI's and bursch muffler (or equiv) might add 4-5hp on a 2.0, 3-4hp on a 1.7.
Also, yes the hangers are different as well of course. The 1.7 puts the muffler higher.