Having trouble with my wastegateS too... Had the rear wastegate tightened recently which got rid of the rattle but now the wastegates aren't closing properly under load.
Car is laggy, exhaust is boomy and generally feels like a dog. Going back in to Woods next week to take the dp's off and try to properly adjust the front & rear turbo and check vacuum pressures are as they should be.
Either looking at a lazy actuator that isn't closing properly, an ill adjustment of the actuator rod or possible faulty vacuum pump not providing enough vacuum to close wastegates.
I suspect its a mechanical fault with the actuator rod (too much play causing the wastegates not to seat properly) in which case if the adjustment doesn't work its new turbos.
Well out of warranty = Not appy!!