Cat Filter Performance: Does it Slow Down?

Hello everyone.
I exceeded 191,000 km today.
Since I've been experiencing problems under heavy load for some time now, here's my question:
Could accumulated soot in the exhaust or, perhaps, a faulty EGR valve reduce the cross-section so significantly that it triggers the limp-home mode at around 3000 rpm in 4th gear and 2900 rpm in 5th gear?
(I still have a software error in the engine control unit that causes it to return to normal operation without shutting off the engine...)
Background:
After a repair on the tensioner for the Lima belt, the guys at VW forgot to properly reattach the hose from the mass airflow sensor (MAF) to the intake manifold. (causing a vacuum leak). During that time, I experienced various issues, ranging from a 'kangaroo' diesel (erratic engine speed) to a loss of power and even the engine going into limp mode.
At that very time, I had just started experimenting with an adjustable chip from KW, which is why the engine was producing power, but with a significant amount of soot production.
Since I reattached the hose, I've noticed significantly less black smoke, although it still smokes when running at full load.
(I have temporarily removed the chip, set it to the minimum power, and tried various other things, so it's not related to that)
Russ is getting around 140 km/h when he pushes the accelerator pedal during overtaking... but that's also when the limp mode sometimes kicks in, or it simply doesn't produce any more acceleration, but just maintains the current speed.
'Since this effect has intensified over the last 20,000 kilometers, but diminishes again on longer highway drives, I suspect a connection with the EGR system or the cross-section [of something].'
What do you think about it?

... if you think it's worthwhile, I would also open up the intake manifold sometime during Easter and take a look inside to see what the cross-section reveals, because some posts mention deposits of 3-5 mm.
But could it possibly be the exhaust?
How long does the Russian filter last?
'Empirical values?'

Best regards,
Sure, here is the translation of the text from German to English:

'Micha'

Hello,

If your engine control unit (ECU) goes into limp mode, what error codes will be stored in the fault memory?

Hello Michael.
there is nothing there.
That's why we've been doing this for so long...
I'm also not sure if it's really the emergency mode; it should have a significant braking effect at 140 km/h if the emergency mode is activated, right?
Actually, it's not really like that.
He just doesn't seem to want to accelerate properly.


greetings
Sure, here is the translation of the text from German to English:

'Micha'

Hello,

Have you ever done a measurement run? You should be able to see there which values are causing the problem.

We did the measurement flight last year.
We then noticed that the mass airflow sensor wasn't working quite as well as it should have been.
If I remember correctly, it was just displaying 60% of the actual target values.
After the exchange, it ran a little better... but not really that much.
'And since I myself can't exceed 178 km/h with the chip (which is set for safety due to torque), I believe the problem is related to soot buildup in the bridge.'
Otherwise, I was thinking about doing a thorough measurement, which is also discussed here in the forum (I think it was Uli who posted it)... but there's one problem:
I have 17-inch wheels with tires of size 215/40 R17, and I have no idea what gear ratio I have.
Oh my.
I don't know what values are supposed to come out.
*ashes on my head*
I think the most sensible thing would be for me to be with my brother http://www.cae-racing.de, and that we should simply take a closer look at the intake manifold...
It's just a 300km drive... you don't just drive that distance on a whim.

bye
micha

@ Christoph S.
Hello Christof.
I just double-checked, as you suggested, which relay 109 I have installed. As I mentioned before, I replaced it last year and now have a new gray one installed. The part number is 1j0...

@ Michael II.
I just came across a post while browsing that expresses a similar assumption.

/viewtopic.php?t=2526

He also suspects a connection to the performance limiter and the reduction in the cross-sectional area of the exhaust pipe.

I will double-check all the hoses around anything related to vacuum pressure, just to be safe.

And maybe someday I'll also replace the intake manifold on my brother's car...
I will then speak loudly again .