From taking my motor apart it looks like we already have a variable intake runner that is vacuum assisted just for this reason. BMW engineers aren't stupid, they thought if this stuff already
At low rpms the intake runner is shortened, at higher rpm it becomes longer.
A TB spacers will not net you any gains, might just cool off your upper intake manifold a bit, nothing really worth making or buying one.
And EuroJDMHB: we already have headers from the factory, not manifolds like the 6cyl motors. the M42 has the best flowing factory header I think I have ever seen, an aftermarket one will give you no power. You might see a few hp with a catback though. Nothing that really justifies the price unless you do something that adds a significant amount of power.