A common way to increase boost w/o changing the wastegate is to introduce
a leak valve between the intake manifold and waste gate. Practically
the line is cut and a "y" is put in. A hose and an adjustable valve
is put on the third connector.

By adjusting the valve, some of the air that would have opened the
waste gate is leaked off, and the watse gate will open at a higher
pressure. Note that this system is difficult to set right, and
requires frequent checking of the boost pressure.

Now, by adding an intercooler, you also introduce more air w/o
increasing the boost pressure. In the TD fuel pump the fueling is
regulated in response to the boost pressure by means of a membrane
pressure meter, where a needle goes up and down into the pump internals.
The curvature of the needle sets the max fueling for a particular boost.

In factury IC equipped cars the needle curve is different from regular
TD's and allow higher fueling at high boosts.

A cruder solution (avoiding getting an IC fuel pump, or modifying
yours) is to just turn up the max load screw. This won't give the
optimal fueling, but maybe close enough.

/Martin