Turbo valve relationship to the turbo and coils

I recently had my turbo replaced and they didn't replace the turbo valve, my mechanic is saying that the turbo valve in no way could have gone bad because of the turbo itself was malfunctioning, they are saying that the turbo valve is completely separate.....
Backstory: Originally the car had an oil sludge problem which was fixed under the warrenty. Because of the loss of power in the engine (due to the sludge problem), the turbo was malfunctioning it was also replaced under the warrenty, but has never actually worked correctly( between 3000-4000 rpms the car BUCKS ) The mechanic says the problem is the turbo valve is malfunctioning and they are not going to cover it, because it is unrelated to the original problem. Also I have had all the coils replaced and 2 of them replaced twice is any of this related....please help me!!