If you say it did boost up to 11psi, then it should be still doing it. You say it stoll creates a vaccuum on the turbo intake, so the wheels are still spinning.

It cis most likely a post-turbo intake leak. Thing like split hoses, hoseclamps not tight enough, split intercooler, split custom intake manifold are all possible causes.
If it's overfueling then maybe it's an ignition or tuning problem?

It's a bit mysterious. I also would think that a t28 was too big for a 1300-1400cc engine.