84,000km sounds about right for carbon build-up depending on the vehicle.

If they have to take the manifold off to clean out the carbon then they should also pull the injectors out (they are just below the inlet manifold) and clean / test them.

As a diagnostic technique you might try chemically cleaning the inlet & see if it improves the situation. It won't clean it out entirely but will get rid of a lot of gunk.

I use a mixture of 2 parts Kerosene: 2 parts Mineral Turps: 1 part Petrol & feed about 1 litre in through the air temp sensor hole while keeping the engine revving around 3000rpm.