If you have a turbo diesel with a mechanical pump (not sure about these new fangled common rail jobs) a good way to tune for better performance without increasing max boost pressure is to adjust the boost compensator.
I don't recommend doing this yourself necessarily, but here is some info on what it all means.
http://www.stonefisk.com/doc-arc/def_man/ThermoGuard_Tdi_Tuning_Rev.2.pdf
It's a very good article and well worth a read.
I think I need to increase the pre-load on mine a a little.
