Mike,
Before you go messing too much with the boost have a read of this thread:
DSE Power!!! increased boost pressure...
The guy Chas is the best guy I've come across on the P38 DSE and has helped me out many times with top advice. He raced diesel Beemers so probaby knows more than most about the M51 engine.
If you do a search on ebay there is are devices for small money (£40) that claim to allow you adjust the boost easily:
RANGE ROVER P38 TURBO BOOST CONTROLLER GAUGE KIT 2 | eBay
I fitted something like that but less sophisticated to an old 200tdi RRC I had. With the 200tdi adjusting the boost was a simple case of adjusting the length of the wastegate actuator rod. I got my 200tdi operating right on the maximum recommended manufacturer setting, but did not go any further.
With my P38 I didn't have to adjust it as when I checked it with faultmate I was already right on the max. I was just lucky I guess but that means that I haven't actually done the P38 myself. Two of my P38 friends over here have adjusted theirs so I know it is pretty straightforward.