Are there any visual differences between Boxster and Boxster S?
 

Or is the only real difference just speed?

The Boxster S has a third opening in the front grill, in the middle. Other than that, a few trim pieces in the interior are all I know of that look different.

yep visually pretty much the center opening in the front apron, (as above)

... the S has cross drilled brake disks (rotors) and red calipers and the S has a 'chrome effect' trim around the winshield (mine doesnt changed it for black as the chrome effect is silver film over plastic which comes off over time).

Don't forget about the dual tip on the S exhaust and the white gauge faces.

The emblem designation on the rear lid.. ;)
and usually the S come from factory with bigger wheel and 6-speed transmission.. and depending on the years some of the option are 'included' in the base Boxster S..

As you've learned, it's not the "looks" that make the S different; it's about what's underneath that increases performance.