[Jules]

I don't think leaking Turbo seals would show any reduced boost than normal.
They only leak a tiny amount pressure/volume wise, but this is enough to produce plumes of smoke from just a few drops of oil per minute.

They only sure way of testing a turbo is by substitution.

A recent local owner here had a Garage "test" his turbo and gave it all clear.
Having replaced everything else mentioned above that would cause smoke, we decided to bite the bullet and replace turbo.

Now smoke free but he's £600 lighter in the pocket!