I think wood trim on a car like the Z4 is just a poor choice to begin with.
It's a sports car, and being so, should only be offered with brushed aluminum, carbon leather, or piano black.
As for the black on brown debate. I think a lot depends on the shade of brown. Dark brown has a tendency to look to dark and muddled. But a lighter brown, with a tint of red and it can look amazing. Take for instance an E46 M3 with Cinnamon on Black or Carbon Black; or the Audi A8 from Transporter 2 (Black with Amaretto)
I think the brown interior adds a touch of warmth and class to the interior.