Interesting. Did you try SailKote yourself? Sort of seems like a sensible thing to do. I'd really prefer not to have this tubing in my doors for the duration of my ownership (it's in there now) and I don't experience creaking and don't know if my car does because I installed the tubing on day 1 of ownership. But if I can go back to how VW designed it and have it stay quiet then I will.
I used sailkote on my MK6 for about the last year I've had it, and I've applied it to my MK7 which I've had for one year.
It works
really well. Amazingly well really, and it lasts a long time.
It's a marine dry spray lubricant, literally used to ease the raising and lowering of sails, and such.
I found out about the product through someone who posted a link from a BMW forum to the MK6 site.
BMW's apparently have the same creaking problem, and the same use of felt on one side. The BMW poster said that they (the BMW posters) had been going about tying to silence their cars by concentrating on the wrong side - the rubber side - of the problem.
Take care of your seals, I always keep mine clean and treated.
But as for making them more quiet, Sailkote the felt.