The older engines had injectors squirt fuel onto the valve, but that is no longer the case with the direct HP injection systems used today. They have designed these engines with 2 intake valves and when they become restricted your engine cannot breathe well.
This is a common problem on Audi and Mini as well.
I have the walnut blaster and proper adapters for the job.
Come on by and have me restore the power you had when your engine was new.