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Boost is not a bad thing. It can and actually does improve efficiency on a diesel in most cases. It is a case of diminishing returns however and you will reach a point where the improvements are off set by the parasitic load it takes to generate additional boost.
Generally a turbo diesel will only make large amounts of boost when it needs it. Off peak load the turbo usually finds a happy equilibrium if its matched to the engine right(Car makers usually do a very good job of this). A properly matched turbo that is tuned for efficiency will actually create more boost than back pressure at cruising speed and load. Unless you know for a fact that its not tampering with your turbo to get it to run at a lower boost level at cruise isn't likely to improve your mileage.
Tampering with other things such as a free flowing exhaust or intake that will increase your boost and will generally help however. Replacing the turbo with a more efficient one will help too. Just keep in mind that all aftermarket turbos are not more efficient. Some are there to just target the performance market.
The only real way to improve mileage by lowering boost is to keep boost low by the way you drive. The turbine of the turbo is driven by energy not recovered by the pistons. Driving in a manner that keeps your boost low is a sign that you are recovering as much energy as possible with the pistons and therefore are recovering as much energy as you can to move you down the road.
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