Replace the valve cover gaskets first. They're probably stock in which case they've most likely compressed over the years.
You may replace the screws with M6 bolts. The philips head screws were used in order to prevent over tightening and make it easier to service. The torque spec is only 22 INCH lbs. Do not over torque them or they will leak again. Also don't forget to put a small dot of RTV in the corner where the #1 cam cap rises up from the head.
Yes, an exhaust leak before the turbo can cause low boot. Another possibility is a weak spring in the actuator. As far as I know you cannot get a replacement actuator... you can add an aftermarket universal if yu fabricate a custom bracket or you could add a boost controller (a manual boost controller can be built very cheap!). Get the exhaust leak under control first.
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