Cylinder Head Machining for Porsche Diesel Engines

A friend of mine wants to work on my cylinder head next week and modify it a bit. However, I've heard from people in the gasoline turbo community that porting the channels doesn't necessarily have to bring any benefits. How does this apply to a Pumpe-Düse TDI engine? Has anyone had experience with this? I don't want him to start working on it and then find out it was all for nothing.

What you want – namely, to get more air into and out of the cylinders faster – can be achieved with less effort through increased boost pressure and a turbocharger.
At most, there might be a slight improvement in the turbocharger's performance and when installing the charger.
Things are different with the suction cups!
Actually, in my opinion, it's a waste of time and effort – for example, when it comes to things like LLK (likely referring to a specific component), air intakes, or exhaust systems; there are usually more worthwhile optimization possibilities.

That's the whole point – just a drop in the bucket. To get that extra bit of performance, I don't need to have it modified. The exhaust system is 70mm from the turbo, and the intercooler is already a different one. Also, the piping for the intercooler has been optimized.

Personally, I see it the same way as teileklaus. But if you already have the engine, I would eliminate the manufacturing tolerances.
Place the gaskets for the intake manifold, etc., over the openings and adjust accordingly. To be clear, this won't provide a significant performance boost of, say, 10 horsepower; it might add at most 1-2 horsepower, if anything.
So, if you don't happen to have a dynamometer in your garage, you probably won't notice any difference in performance; at most, you might perceive a change in dynamics or fuel consumption. However, the latter is often more subjective and influenced by personal perception.
In high-quality tuners, roughly speaking, not much more is done. Of course, channels are expanded and refined, etc. Ultimately, the transitions (e.g., intake manifold/carburetor) are adjusted. "Tuning" essentially means "adjustment." Adjusting these transitions is more of a fine-tuning process, the final touch.
Best regards,

We're also going to stop making any further modifications; the transitions from the intake manifold and exhaust header have already been optimized.

"And I definitely wouldn't cause any cracks there with just 1 or 2 horsepower."