First off I think you are dreaming on that rear tire. The stock ex500 has a 130/70r17 tire. The larger tire looks better, but depending on teh specific tire you choose, you may have a hard time fitting even a 140/70r17 or a 150/60r17. Even then with these larger than stock tire sizes, depending on the model you buy can rub the swingarm, chain and other parts back there.
I can pretty much guess that you wont get a 170 tire on that rear rim, and if you do you will be hard pressed to get it to seat right, and it will be very squeezed giving you a pretty extreme profile and very wierd handling.
For the muffler question, you won't loose any noticable performance, however depending on how long it is and how much back pressure you have, you might have to re jet the carbs. If the carbs were already jet'd for the yosh pipe you should be good.
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