The MAP sensor on an MFi RST is inside the ECU. There will be a vacuum pipe to it.

What you are referring to is the Amal Valve.

It sounds to me really like a fault with the turbo. I think the reading you see on the gauge where it jumps from nothing to 20psi is a fault with the gauge, like a sticky needle. The car does feel laggy though, so I am still with the idea of it being a faulty turbo.

Yes, the fuelling will need checking again once the turbo issue is resolved and should also have the actuator properly set-up, as said above as potentially, by increasing actuator pre-load, you may end up running too much boost and melting the engine. Especially if you are relying on a boost gauge that really looks as though it is faulty.