Early W2's and real-world performance gains?

The problem here is a few things.
Besides a lack of info of what you have now to where you want the go is;

The dual plane is a copy of the old stock one. While it may flow better, it is still limiting.

The as cast W2 head isn’t the best flowing head. But it does perform (or should!) about as good as a well ported stock head. (Not fully ported!)
Exhaust manifolds would further hamper performance efforts. You would have to be crafty in your engine build & camshaft. It is possible!

Flow test the heads to see where they stand so you can get a little inside info for selecting a cam.

A call to a cam company for a FAST type of cam would be a good start.
I do like your idea though. A big stick Disguised as a small toothpick.