I think your dealer did tell you something wrong, the Racing Kit of the SL contained the big Y-Ti-Collector w/o flapper valve, see images below....
Yes, indeed they did...as I was typing that last post I immediately thought of having a picture of what was in the box that came with the SL - funny how genius minds think the same
It would now seem to me, that the $3855 price tag is what's in that box "how sweet it is"
Add on a carbon subframe & carbon body work....different wheels and that might just be the SL (Gecko/Jogy, what do you say, what differences are left?)...with the 2015 R being the SL for the masses ---- "ooo, a red one.....I'll take it" !!