It is a doubler, just with the bw1350 as the front case and the d300 as the rear. The "traditional" rbv doubler is with a bw1350 as the front case and a bw1354 as the rear. The d300 is a stronger case, has deeper gearing, and has the capability to be twin sticked - making it a better option. That's what I've gathered from that post anyways. If its a drivers drop case then you'd be in business, but I guess you can flip those cases without issue so either way you're good to go.
I've got a doubler in my dd. over 20k miles on it and still truckin - everyday.