and those stockers are a lot better than what typically passes for an exhaust manifold
I think that may be marginally true. But most gear head/tuners would agree with me when I say that they both provide significant restriction at the merge point/primary collectors. The down tube merge point/secondary collector is also restrictive on the stock exhaust.
All 3 constrictor points must be opened before you will significantly improve flow. Otherwise, might as well stay with the stock manifolds. Without modifying the downtubes there is still going to be the same traffic jam at the secondary collector.