I have a stock '88 ST165 and the same boost gauge as shown in your picture. It was fitted by the previous owner.
The nylon vac hose is connected to the brass tit in the end of the intake manifold next to the vac hose connection for the VSV etc.
On full throttle/power the gauge only reads about 3psi (that is two didget marks before the 5 on the top scale).
Now the gauge is either hooked up wrongly or I have a problem with the turbo system.
The turbo is fine with no damage to the impellers and no play in the shaft and the engine is healthy. The TWG is operating ok with in the factory specs. That leaves the turbocharging pressure VSV or the T - VIS VSV or turbocharging pressure sensor to be the problem. No I don't have any vacuum leaks.
The questions I ask here is (please) are:
Is the gauge hooked up correctly?
How can I test these bits (VSV T_VSV etc, etc,)?
Better still how do I get around the TCP VSV and sensor and hook up a manual controller?
I would like to raise the Turbo Pressure to about 10psi. No more than that I think, as the motor is original and has done 160k's (until a couple of weeks ago had the original factory Denso spark plugs were still fitted)