The actuator controls the action of the wastegate opening & closing it to control boost. The actuator must be preloaded to ensure the wastegate is held shut until the desired pressure is reached. Shortening the actuator arm will increase the preload on the actuator, hold the wastegate shut longer & in turn increase the boost up to a certain point after which an actuator with a stronger spring needs to be fitted for higher boost levels.
About 6mm or so is usually enough preload.
The standard Metro actuator gives 4psi of boost {the ECU giving a max of 7psi}.

Raising the boost.
The Metro Turbo as standard runs a max bost of 7psi, and on the standard compression ratio of 9.4-1 is pretty much on the limit as far as detonation occuring is concerned.
To be able to run more boost it will be nesassary to, either lower the compression,
install an intercooler or both, depending on proposed power output. The fuelling and ignition must also be spot on to ensure detonation doesnt occur.

An uprated actuator can be used as above, or a bleed valve can be fitted.
The uprated actuator may give a slight advantage over the bleed valve. Due to the nature of their design theyre supposed to stay closed until the set boost pressure is reached, and then open quicly. The standard item may open slightly before the set pressure is achieved. So some of the gasses will bypass the exhaust turbine. So the Turbo is not working at 100%. The uprated actuator may provide a little more torque, at some points.