I assume that when you talk about an R40SC you mean "why are the R40's so flat?"
The simple answer is: while the curved spokes are more beautiful to look at, the most efficient design in terms of strength and weight is a straight beam, especially in buckling, which is the mode of loading that the vertical spoke is seeing (and bending when the vehicle is in a turn). Any curved or extra material is only adding weight. The really tall backpad is required to reach the low offset that the M3 is able to run, so the gigantic bowl in the center is designed to remove the mass that is not required for strength. The R40/43 is designed first to be light and strong, and then second to look good. As far as i know, they are lighter than the Apex wheels also. Keep in mind that HRE could easily make these wheels weigh 15 lbs, but they are very conservatively load rated for our heavy cars to go around the track for years. Most race teams throw their wheels away at the end of the season, but that would be unacceptable for the consumer market.
I have the R43's and would certianly like them to be a little more concave...however, when i look at them the beauty is in the engineering for me.