[oil pan 4]

The only time you want to bypass turbine drive pressure is with the waste gate. Use the waste gate to limit boost pressure.
The supercharger is rpm dependent. In an old 2-stroke Detroit diesel the blower is setup to feed the engine an average 2 pounds of boost, through the entire operating range.
If you want more boost, you have to add a turbocharger.

I have ran a few diesels with no exhaust manifold or only an exhaust manifold. They are not as loud as you would expect.
Right now I run the my 6.5L with a straight pipe coming off the turbo.

Gasoline engines are most efficient at full throttle because you eliminate isobaric losses. Isobaric losses on a small 4 cylinder gas engine can be around 2 horsepower at cruising speed. V8 engines have even higher isobaric losses.
This is the entire reasoning behind the cylinder deactivation the OEMs are using now.

Think hybrid engines are something new? Nope. The turbocharger is another engine. Turbodiesels are the original hybrids.

On the 1.6, just run a 2 or 2.5 inch after it comes off the back of the turbo. You want as little restriction as possible after that turbo. I believe the factory down pipes on the K03 and K04 turbochargers are 2.5 inch. The KO3 and 4 series uses its own down pipe setup, the waste gate and turbine discharge dump into a shared plenum and the plenum is like 4 inches in diameter and then reduces down to the smaller pipe. Any down pipe off any car equipped with that turbo will work, you will just that plenum and reducer. Chances are you will have to cut the down pipe a few inches after it reduces down.