Are there any downsides (besides wire length) to relocating the AFM before the throttle body and aftermarket BOV before the AFM?

 

are you talking like a blowthrough setup, whereby the AFM is located after the turbo? The stock ECU may not like it too much. You will at least need a fuel controller. And an S4 AFM needs to be mounted level or it will read funny and throw the ECU off.

I know people do that on other cars (3000GT comes to mind) and it definately changes the tune. The AFM is now measuring compressed air.

 

its been done many times. i will not search this for you, but it seemed to work just fine.