My interest in Magnum manifolds did not concern wide open throttle peak horsepower.

I was more concerned about all 8 cylinders exhausting about the same
so that on the part throttle intake stroke
all 8 cylinders would have nearly the same “balanced” air to fuel ratio.

My fuzzy memory is that the 1992 press release about the then new 5.2 Magnum V8 specifically said “new balanced flow exhaust manifolds.”

Then in 1994 Chrysler “choked up” to a smaller size the exhaust manifold exit diameters on both the 5.2/5.9 v8s.
My guess is that this was done to create more flow into the EGR valve to meet tougher NOx emissions limitations from 60% throttle to 90% throttle opening.
Remember that back in those days the “Congressman John Dingell passing power loophole” was still in the EPA law and EGR could be turned off above 90% open throttle.

When Dulcich did his many dyno horsepower tests on Magnum and other manifolds 0n a MP Crate V8 he “went the extra mile” and tested the two unique driver/passenger Magnum designs which are not symmetrical. Dulcich found one side to flow slightly better. This makes sense because the rearmost #7 and #8 cylinders seem to be “more challenged” in how they merge into the flow.

found his nonMagnum comparison, but not the other one....

https://www.motortrend.com/how-to/mopp-0307-mopar-crate-engine-exhaust/