For all its new body pannels, the BS is still an old architecture (after all, it is still a current generations SL). I do not see the reason for all the complaining that it has an old engine--which happens to be one of the most advanced in production today, nevertheless--and an old transmission--again, one of the most capable and revered automatics in production.
THe SLC is getting all the goodies, with an 8 (eight, not seven) speed MCT and the new 8 cylinder engine both in NA and BT form, not to mention a design to rank it as an aesthetic, sensual and "bespoke" competitor to anything the Modena, Italy factory can roll out.
The BS is simply one way for the R230 to bow out with a serious bang. I suspect that, when disclosed to the public, the actual outright investment in the project will be relatively modest as new model developments go. It is, after all, a tuning shop job.
ON the other hand, we should expect more when the R231 bows into the scene. The R231 Black Series will undoubtedly have far less compromises.