[oil pan 4]

Water injection puts more air into the cylinders by cooling down the air. You want to get the most air into the cylinders with the least amount of energy expendture.
Water injection is a very cheap way to get more air into the cylinders with very minimal energy input.

Water injection can raise boost with pre-turbo spraying (which I don't really recomend), but that is usually done under heavy load.

Every diesel should have an EGT and boost gauge. Also fuel feed pressure wouldn't hurt (put a gauge between your last fuel filter and before the injector pump).

It sounds like your Benz has a fairly restrictive exhaust housing. There is no need for a car with a diesel engine that big to run 9PSI just to roll down the highway at 70mph.
That measns the turbo is doing a lot of work when it doesn't need to be.

The dodge guys claim that keeping your numbers below the 600-6 mark is the way to go. That means keep your EGT numbers under 600'F and boost below 6psi.

I run about 1psi of boost at 55mph and up to 3psi at 70mph. My EGT numbers tend to be a little over 600'F if I am rolling along at 70mph.