I think when I've written it, it's been that it will give you 90% of the benefit of headers.
The stock manifolds, when ported, are sufficient for those that are not struggling for that 99.999% of performance out of the engine.
But when you get guys like those racing for the top 10 on the SCCOA fastest list, 90% just isn't enough.
I don't know if Jim has posted his HP and Torque numbers yet, but I'm sure they are well over 300hp.