You turn it up by shortening the actuator rod - undo the locknut and wind the shaft up to shorten it....but don't do that yet.

The boost should come in a lot earlier, which is why I said the actuator's lazy - if it's making the right amount boost, but higher in the rev range, it could be because the actuator's not holding the wastegate shut for long enough, perhaps because the spring in the actuator's worn.

Boost is cut by the ecu (assuming your amal valve is still connected) when it thinks there's too much. If it were a cossie, I'd say to check your map sensor, and its hose, but there may be other things to check on your RST.