Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
If you haven't already then check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
Also check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.
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According to my information the wastegate actuator is the old fashioned pneumatic type but if its the newer electric type then you can check the actuator is working by ensuring that you've pushed it fully home and then with your assistant watching the actuator turn the ignition on but do not try and start the engine
the actuator should move in and out while it 'learns' it position