Good point, Daox.
Even the Prius alloys are fairly chunky, as is the style these days. Though the compromise is the inset trim ring, which makes sense since the outer (and top) portions of the rim create the most turbulence.
PM: I found a larger picture with more detail of the Lexus rims. They have an odd spoke shape, and it wouldn't surprise me to learn they're optimized to minimize drag despite the open design.
Pic: (2000+ pixels W):
http://caraphoto.caradisiac.com/r/e/...H/HR_71271.jpg