A few thoughts...
1) I agree with
Boxter that the 987 S rims and the similar looking Carrera S rims look better on the white 987s than the Classics, perhaps because they're more substantive against such a washed out background (and I actually dislike the CS rims despite having them). If anything I'd get the 19' Sport Design even though they look ridiculously painful to keep clean.
2) In case you hadn't checked, the difference in weights between your wheels and Classics are:
987 18" S: 21 - 11 and 22 - 14.
19" Carrera Classics: 23 - 10 and 26 - 8.
Not overwhelming, but not trivial either. If you remember a forum member named Smith12895, we used to kid him about his 987 base being slower than other 987s on account of his Carrera Classics.
3) If you haven't, you may want to comparison shop with Wheel Enhancements, in addition to Wheel Dynamics.
4) If you want to make a knock-out visual enhancement on the white Boxsters, I think getting your roll bars painted (if it can be done, and I don't see why not) would make a much greater impact than the vents and wouldn't be much more $.