I understand your point entirely.
My point is, I make manifolds, dumps and exhausts for a living, and I would simply use the stock manifold for the power you are chasing. If you wanted 400+ I would be steering you towards a HKS or steampipe. Take my advice or not, it won't be keeping me awake at night.
There are always gains to be made, I agree, but it comes down to cost per KW, and manifolds usually add so much cost, not just in buying the manifold, but fitting challenges with highmount setups, compromised angles on the dump bends, wastegate challenges... Remember, I have fitted all types of manifolds before, including twin gate split 6Boost setups, they never install as easily as you would expect.
Here is my favorite RB setup which had it's own challenges. Wasn't cheap either, but the results were great. If you plan to head down that path, find a decent fabricator first, then speak to a few manifold suppliers. Some are more approachable than others.