Good thread going here.
It looks like Pypes sells two types of downpipes.
One with the flanges (DS10S) and one without(DGA20S).
The set without flanges are not necked-down and would have less restriction but it looks like they will only work with RA manifolds:
http://www.pypesexhaust.com/downpipes.html
I suppose we cannot get these for Logs because the sharper radius requires the flanges to be pre-installed.