More boost raises engine efficiency. Had to find it in the internet archive but here it is and it says the same thing as the earlier papers (From 2004 on a Direct Injection engine).
http://web.archive.org/web/200611080...4/A6_1_022.pdf
As far as efficiently driving a turbo diesel engine you want to drive generating low boost pressures. That shows the pistons are extracting as much power as they can before they dump the rest of the energy in the combustion gases to the turbine.
As far as the engine itself you want it to generate the most boost as efficiently as possible. The purpose of the waste gate is to keep boost pressures in a range to avoid wear and tear on the engine and keep the boost in an efficient range of the compressor map and keep back pressure in check.
So, if you are trying to increase the boost setting on your current system you probably won't improve efficiency. If you improve it by using a more efficient turbo you can raise boost and get better fuel economy