More boost raises engine efficiency on diesel though you get diminishing returns as it increases.
http://web.archive.org/web/200611080...4/A6_1_022.pdf
The question of whether more boost will help on a particular engine depends on how much more energy it takes to create the additional boost, the efficiency of the system doing so, and the increase in friction due to running at higher pressures (note that the above paper showed efficiency increases all the way up to five atmospheres).