Direct injection means the fuel gets injected directly into the cylinders after the valves instead of into the intake air stream ahead of the valves. The fuel never gets a chance to clean the intake valves on its way into the engine. These types of engines commonly have carbon buildup on the intake valves that must be cleaned up to restore performance.
No amount of Top Tier gas or cleaners in the fuel tank will do anything to prevent or fix this problem. The fix is either removing the intake manifold and removing the deposits with a media blast, chemical sprays and manual scrub, or a chemical spray into the intake stream while the engine is running, soak to loosen and hard drive to blow the deposits out the exhaust.
The intake treatment could have been done much earlier (maybe every 35k miles) to melt the little bit of buildup off rather than waiting to 100K with a performance fall-off.
But you also need to make sure the PCV system, the turbo seals and everything else is working properly to avoid having a large buildup again in short order.