When I had mine done I noticed an improvement. So much so that I plan to treat it as service task that I'll have repeated when a borescope image indicates that it'd be worthwhile. Every 2 or 3 years may see something to clean off without letting it build up into a heavy crust.
I personally believe that these engines benefit from being 'driven' rather than crawling around at low rpm for too much of their life, 12 month servicing minimum (I do low mileage and some short journeys to work and home so I slip in an extra oil only change as this kind of use could be termed as 'arduous conditions' according to the hand book) , good quality fuel of 98RON minimum (as the sticker in the filler cap says) and the best engine oil such as an ester based one which has a 'clean' fully synthetic base rather than your regular synthetic oils. This is worth a read
http://www.bobistheoilguy.com/esters-in-synthetic-lubricants/I do track days every now and then which helps to keep the lungs clear.
Below is a direct quotation from the technical staff of VAG with complete acknowledgment of the FSI intake valve deposit issue, and it's impacts, including: decreased performance, misfires, catalytic converter damage ... etc.
"Gasoline engines with direct injection of the fuel into the combustion chamber, i.e., not into the intake port, suffer especially from the problem of the formation of carbon deposits on components. Carbon deposits form especially in the neck region of intake valves. A more exact analysis of how these carbon deposits form leads to the following result: Oil and fuel constituents first form a sticky coating on the components. These constituents are chiefly long-chain and branched-chain hydrocarbons, i.e., the low-volatility components of oil and fuel. Aromatic compounds adhere especially well. This sticky base coating serves as a base for the deposition of soot particles. This results in a porous surface, in which oil and fuel particles in turn become embedded. This process is a circular process, by which the coating thickness of the carbon deposits continuously increases. Especially in the area of the intake valves, the deposits originate from blowby gases and from internal and external exhaust gas recirculation, and in this process, the blowby gasses and the recirculated exhaust gas come into direct contact with the intake valve.
Especially in the area of the neck of the intake valves, excessive carbon deposits have extremely negative effects for the following reasons: In the case of Otto direct injectors, the successful ignition of the stratified charge depends to a great extent on the correct development of the internal cylinder flow, which ensures reliable transport of the injected fuel to the spark plug to guarantee reliable ignition at the spark plug. However, a coating of carbon deposits in the neck region of the intake valve may interfere so strongly with the tumble flow that ignition failures may occur there as a result. Under certain circumstances, however, ignition failures can lead to irreversible damage of a catalytic converter installed in the exhaust gas tract for purifying the exhaust gas. Furthermore, the coating of carbon deposits in the neck region of the intake valve causes flow resistance, which can lead to significant performance losses due to insufficient cylinder filling, especially in the upper load and speed range of the internal combustion engine. In addition, the carbon deposits in the neck region of the intake valve may prevent correct valve closing, which leads to compression losses and thus sporadic ignition failures. This in turn could irreversibly damage the catalytic converter. There is the potential for small particles to break away from the coating of carbon deposits in the neck region of the intake valve and get into the catalytic converter. These hot particles may then cause secondary reaction and corresponding local damage of the catalytic converter. For example, a hole may be burned in the structure of the catalytic converter.
Globular deposits are found especially on the valve stem downstream from a partition plate in the intake port. Due to the dripping of high-boiling hydrocarbons from the partition plate towards the valve neck or valve stem, globular carbon deposits eventually form there by the sequence of events explained above. These deposits on the valve stem can result in flow deficits due to undesired swirling and turbulent flow around the globular carbon deposits. This may persistently interfere with the formation of stable tumble flow from cycle to cycle.
A possible solution would be to keep these sources of deposits away, for example, from the intake valve, by completely eliminating exhaust gas recirculation and the introduction of blowby gases into the intake port. However with the combustion behavior of modern reciprocating internal combustion engines, at least external exhaust gas recirculation and the introduction of blowby gases into the intake port are absolutely necessary for reasons of emission control and fuel consumption, so that this approach is not possible. "
