Firstly ... you need to take more beer with you when you go fishing. Fishing is for drinking and sleeping and getting to wee in bushes ... you really need to switch your brain off and relax.
The Walther LGU has a small port as standard. They probably use the same port diameter as they use for the German 6fp rifles. So when they put in a longer spring ( for UK 12fp ) or an even longer spring for unlimited markets, too much compressed air is trying to be rammed through too small a port. Some folk say this has overstressed new springs.
Opening up the port for UK 12fp improves the shot cycle.
This is probably a similar story with the Weihrauch 77/97. They used a port of 2.9mm. That's fine for low power leves and especially if using very light pellets ( sub 7 gr ) for paper punching at 10m. Using a pellet around the 8gr or above weight and the rifles improves by opening up the port ( up to @ 3.8 - 4mm ).
Lots of people have tried experimenting with this. Like Scooby said there was the Whiscombe experiment. Jim Tyler and others have tried it using the drilled out grubscrew method. Some have drilled a hole at the top of the action so that a grubscrew can go through and hold in non threaded inserts. Others have drilled out and tapped the port area and then screwed in drilled out grubs.
I think you mean using a proper Venturi screw ( that always remind me of trying to set up the Weber carb on my old Mini Clubman ) where the screw could be moved in and out to adjust air flow.
This is fine in a PCP where the air flow is pretty stable and constant ( ish ). In the piston/transfer port rifles there's a massive blast of air going through that port. So I don't know how something in the line of flow would affect the consistency of the output to the pellet.
I opened my 77 port from 2.9mm to 3.9mm and using 8.4gr pellets the power went up by over 1fp and the shot cycle improved.
If you read some of the articles by Bigtoe and Wonky Donkey they have pretty much got the piston weight, stroke, spring size and port size worked out for different power levels and pellet weights ( pellet fit and breech design also plays a part ).
Your idea sounds great, and has been done by physically changing the size of the hole ( grubs /inserts etc ), but I don't know if a Venturi screw would work.