Old thread, but I'm running a B5 A4 with the AFC 12v and have about 238k miles.
You're right that there's no ECU mods which is very frustrating. However, there are a few well-established breathing mods that do help wake up the engine a bit. Nearly all the mods out there are "old school" kind of things which involve porting & polishing and custom-fabricating parts to improve the setup.
I do hear that the AAH cams are a nice improvement. There's also adding a velocity stack in the airbox where the MAF bolts up (it's a nasty sharp 90 degree edge).
You can also port the throttle body, but in my opinion the intake plenum (the black plastic duct between the throttle body and MAF) is horrible for flow. It necks down from 3" to about 2.5" and has a very sharp turn. I'd probably look at dealing with this before porting the throttle body.
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