The 5-speed will be fine for flowing traffic. Stop-and-Go traffic sucks in any manual car.
I would consider the speed that you would be averaging. Because if you are going 35mph most of the time, then you might be right between shifting from 2nd to 3rd, which can be annoying.
Traffic sucks in general, but it is completely doable in a Boxster. It is worth it, just to go driving on twisty roads on the weekend.
