In my humble opinion, I believe that you are over-thinking the headers.
Is this to be a no-compromise track-only car, where every last bit of power needs to be coaxed from the engine?
If not, I sincerely doubt that one header style vs. another will amount to any performance differences whatsoever, especially if your existing OEM exhaust manifolds are the Euro spec 3 tube into 1 style (as contrasted against those USA spec models which include the built-in cats).
But if you do have the USA spec built-in cat manifolds, then you may want to give this thread a read:
For Sale: Hollowed OEM Exhaust Manifolds, Complete with Mini-Cats
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