I got my B5 S4 with 180k miles back in 2010. I am the third owner and car has many HP parts including APR 3 chipset and K04 turbos. Now at 244k miles and though it has some oil leaks and a coolant leak it still can do aorund 0-60 in about 4 secs and will hit almost 29 mpg if I keep it under 70 mph. When I first changed the plugs after getting it I put in the NGK iridiums and after getting some misfires regapped them to around .028. That seemed to help, but if I really got on it there would still be some stuttering so I didn't push it and see the blinking light in dash. Well yesterday I changed the plugs for some copper NGK BRK7Es and immediately noticed a smoother idle and better power off throttle. I decided to check the gaps on the old plugs for reference. These iridiums had somewhere between 25k miles and that should have been well within their usuable lifespan. Well here are the gaps that I found after pulling them: Cyl 1: .035 Cyl 2: .035 Cyl 3: .040 Cyl 4: .040 Cyl 5: .040 Cyl 6: .034

This tells me that the iridium plugs aren't lasting as long as predicted, and since I'm getting better performance now I will never use them again! I was thinking of going with the Bosch side-fires, but have heard about major failures that caused damage and I can't afford to take the chance. But I thought I would share this with all of you since there always seems to be a debate around iridium vs copper. BTW, I gapped them to .025.

 

That's some good info. Thanks a bunch for sharing. Any chance you looked at your coil packs too? I know Audi had some issues with coil packs through 2008 or so. Chances are that there might be some issues with yours unless you replaced them recently.
I had a 2002 model and after changing those bad boys out it felt like the car gained 50hp over night.

 

I have not had coil pack issues, but did have to replace an ICM due to misfires. I ended up getting a used set of coils and ICMs on eBay and just replaced both coils and ICMs. I did try the change to the newer coils and ICM deletes kit but had some misfires so sold it at a loss. Wish that had worked out as I liked the setup (No ICMs) and newer coils that can have larger gaps. I even ordered a set of plasma Okada coils but they never came so canceled the order.