My personal experience: I used Seafoam every 10K miles for 60K miles on my car. I never had any bad issues using it. I did get a flashing CEL everytime while doing it. And one time when I had a helper pour in the Seafoam while I revved the engine I smelled rotten egg smells (heating up the cat too much). We stoppped immediately and poured it in more slowly. That's the worst thing I've ever had happen with Seafoam. Did doing it help? I don't know.... Honestly, I still felt like between 80-100K I was going to need to tear it down and manually clean it all out. So, I guess 6 x $9 is what I was out.
That being said, I've read that FORD does NOT recommend using Seafoam or ANY other induction cleaning methods in turbo charged engines. They said that it heats up the turbo and burns the turbo's out. I didn't have that issue and am unaware of VW's having that issue... but I always thought about it while I was doing a Seafom on my car.
I changed my plugs out with BKR7EIX at 55K. No noticeable change in performance. I changed my coils out at 60K and I noticed a difference. I noticed slightly better throttle response and the engine overall seemed more peppy... like the day I got it brand new. Nothing too major, but noticeable.
Also, I thought that iridium was the best way to go for non-meth and platinum was the way to go for meth setups. But maybe I have that backwards.
Fuel filter was more of a maintenance thing and after I did it once, I figured once every 40-50K is sufficient.