Seems to me that if it takes variable length intake runners to wring power out of the engine, any static exhaust manifold will be a compromise. Best overall vs better power down low OR at revs. Headers might shift the compromise point, hopefully towards what matters more to you. You’re going to need lots of dyno and track time to convince anyone there’s more power at the wheels vs “it sounds faster.” The latter seems to be worth money to a lot of people.
05 E320 wagon 197K miles