I'll try to answer in order:
1. I have a Supersprint cat-back exhaust. No chip
2. My average MPG over the last 95 fill-ups has been 29.28. Not that I've been keeping track
That includes AC and wide tires during the summer and about 8 autocrosses a year. Performance is more subjective. Honestly it's been so long since I did it that I don't remember if I really felt a difference or not.
3. There may be slightly more engine noise than stock, but again I don't remember. It's definitely tolerable. I suspect after a few days you wouldn't even notice the difference anymore. At cruising speeds I would say it's not noticeable at all. The road and wind noise are certainly louder than the engine at steady speeds. If anything you'll notice it if you're really getting on it from a stop. Part
#4 is a resonator to keep the engine noise down. It probably does more than the foam could ever hope to do.
4. You'll see when you get in there, but I don't think the foam was for anything other than sound. If you were really careful you could probably retain the foam and the plastic cage that holds it in, but if you read this you'll see there are good
reasons to get rid of it. What's really scary is that there is more of it in the lid of the airbox upstream of the filter. I just remove mine a couple of weeks ago. I'd clear that out while you're in there.
The only thing about this mod that is a hassle in any way is that it seems to collect more dirt in the bottom of the box. I'm assuming this is because of the direct path from the front fascia into the box. It's no biggie. I've cleaned it out once since I did it and there was just a little dirt and some small plant debris in there. It sounds worse than it was and it was all clearly stopped by the filter. I would imagine that the M42 equipped cars have this problem too since that intake path is stock for them. No biggie. Hopefully sometime soon I'll get the Dynamat on my airbox. I'll try to remember to get some pictures and put them in my gallery. I'm really interested to see how that will affect the intake temps. I'm really hoping it helps with the rise in temperature while the car is hot and not running. I'll keep you posted.