If I understand your question correctly (and am not mistaken), your boost compensator will be "seeing" 10 psi, and will enrich accordingly - that is to say, not much. That said, seeing that in a diesel, boost is related to fueling quantity, what you've unwittingly or wittingly created is a rudimentary boost "controller".
When you reduce the pressure on the boost compensator down to 10 psi, it will inject according to 10 psi. This will slow down the turbo, until it is producing just enough pressure to cover the difference due to leakage in the pipe.
If you close the leak, the boost compensator sees the full pressure, say 10.1 or 11 or 12 psi, then it increases fueling, which increases boost further, more fuel, more boost and so on until the waste gate puts a stop to the action at 18 psi.
@all others - please feel free to comment.