S muffler vs. base muffler
 

Do they have a different sound or is it simply that one has dual tips and the base has a single outlet? My car is a 2.7 if that makes a difference.

No sound difference to me. It's just a different tip.

I agree.
I swaped the muffler on my 2.7 for a factory boxster S muffler and there is no difference in sound, it just looks better than oval tip in my opinion.

There is no difference between same model years up till 2003, where there is a change in tone which is more aggresive and throaty than the previous years. Starting in 2003, for those exhausts, the tips were removable. Prior to that, the tips were welded on the mufflers.

The mounting is different between the 5 and 6 speed transmissions so you will need an adapter to fit an s-muffler to a 5-speed transmission.

Well actually.... The 97-99 Boxsters had a different mounting method on the top bracket of the muffler than the 2000-2004 Boxsters- so yes, if you bought a 2000+ muffler to fit on your pre 2000 car (whether it was a base or S muffler), you would have to buy the adapter bracket to make it work. However, if you have a base 2000-2004 boxster, and wanted to fit an S muffler of those same years, they were interchangeable, and no modifications are necessary to fit an S muffler to a base car.

Thank you for the information, I will go ahead with my pedro mod on my base muffler then. I hope by this next week to have it installed and the Stebro race muffler will be for sale.