D13's and D24's are still produced. It would not be to hard to bring back the B6, C6, and C12 loads... just take a standard slotted grain and cut it in shorted pieces (that is assuming you slot the grains before you cut them). The C4 would require some retooling, but I think that would be worth it to also make D8 loads. That is a definate must in my opinion... I have shreaded to many Estes (and Fliskits) rockets with the D13... I would have several packs of those in my range box if they were ever produced.... I love the 24mm D9, use them all the time... but its 24mm....
As for the nozzle... how much does the 18mm nozzle cost on the RCS price sheet? .75 each? Actually, it is not bad if they have the same price as the bp counterpart.... I will still use them. I am not sure, but I think even a C load in the RMS 18/20 case is still lighter then the a BP C Motor... and you get a full C out of it. Not like the 7.5 or so n/s you get out of an Estes C...