The Carreras are considerably lighter than the Turbos at 18". Something to consider. I dropped 100 pounds by picking these wheels and my GT3 seats.
The Braile battery should save another 50 pounds. That's like dropping an extra passenger for me.
The Carreras are also very sought after not just by 911 and Boxster folk but 928 as well. when you sell your car put the 17" stockers back on and you'll have no problem selling the 18" if they're good condition.
I think the silver caps give a nice subtle look