The max boost is limited by the ecu (unless you really mess it
up to the point where it can;' control he system). Changing the
wastegate as you described seems to allow the turbo to idle at
a higher rpm and you get faster boost response (less lag) but
no additional power. Over do it and the throttle goes from a
linear "ramp" to a toggle switch. The correct setting varies
by year (according to volvo specs) and I bet a lot of them are
not set perfectly from the factory. IPD (www.ipdusa.com) recently
had an article in their newsletter on the procedure for correctly
setting it. They might have it online.

To add more boost you need to change the ECU that limits boost
(chips sets do this) or somehow fool the ecu into thinking it is
seeing less boost then it really is.