There's two kinds of performance enhancement as far as I'm concerned. There's the "I just want a little more out of the stock engine" and then there's real modification (re-engineering).

The first rule of modification is, take your wallet out of your pocket and throw it out the window...







Stock 400E engine setup looks like a fantastic high performance base to me. Get a load of that stock intake manifold...that off a race-track or what! I'll say one thing for Merc V8's: they love their short inlet runners. Le Mans homologation mad, I'm sure, which is odd since they're hardly ever there anymore.
Between that and those head castings it looks geared for 4000-8000rpm straight out of the box.
This is a detuned 500E engine I take it.

Due to the capacity and engine base you'll need custom fabricated exhaust. They have to be a relatively small diameter (50-150thou larger than the port diameter if extreemly free flowing), but still tubular and tuned lengths, with a free flowing system. Don't even bother with 500E aftermarket jobs even if they bolt on, I'm certain it'll have much larger exhaust ports and the corresponding headers will lower gas velocities on the 400E too much and kill your power curves.

You need the heads either flow benched or computer modelled (any high tech modification or race quality workshop can do this), you've gotta have your Mach numbers and flow coefficients sorted from any less than performance orientated detuning efforts on the original engineering. They'll have to model the whole engine setup to find out what its requirements are (standard practise for places like AMG, who'll then check out projected kit combinations on a dyno).

You'll want to relocate the MAP sensor if it's a restrictive type and somewhere ridiculous (ie. in the intake path), by getting a custom, non-restrictive one tapped into the intake manifold.
Install low restriction filters into your air cleaner boxes, the reason low restriction types may not have worked for some home modifiers may have been due to a restrictive MAP sensor which kinda defeats the purpose of upping the cfm flow back to the throttle body's capabilities elsewhere.

Definitely up your diff gearing. 2.6 is ridiculous for line acceleration and a 2.8-3.0 ratio is much better for an auto. The improved breathing ought to improve economy to compensate and you can always go for a further overdrive transmission if you're that pedantic.

Lastly, *after* you've done these mods get a custom chip mapping. Fill the tank with the typical fuel grade you're going to be using and sit the car on a specialized dyno and get those spark curves and shift points mapped for your personal setup and vehicle characteristics.

All this will take the engineering specifications back to their projected optimum (ie. a huge increase over production detuning). From there you can actually start modifying for increased flows, cfm, valve tuning, etc. and start chasing 500hp.

You may or may not want to sell your wife's jewelry.