I know several who have tried this mod. None found it worth the effort
in terms of added performance (that is to say, there is no discernible
improvement although, to be fair, nobody I know has done before/after
dynomometer tests either). The main problem, I should think, would be
that there is no way to get around the fact that the intake plenum
casting sitting atop the V-6 engine has a 2.5-inch inlet opening and
the BMW AFM has a 3-inch outlet. You CAN soak your boot in hot water
and get it to stretch over the new 3-inch BMW AFM outlet throat, but
it's still going to 'neck-down' to 2.5-inches on the other end. Which
means that you really aren't improving airflow as the volume of air is
restricted to the size of the narrowest opening in the system which is
the 2.5-inch throat on the plenum chamber inlet to the throttle body,
all else being equal. But what you will have to do is reset your idle
speed and your idle mixture using the aforementioned air/fuel mixture
gauge.
This overlooks the fact that the narrowest opening in the intake
system is inside the airflow meter--considerably smaller than the AFM
outlet. I think the interior of the big BMW AFM is still narrower
than the stock Alfa intake hose. I'm just reluctant to take the
stretching approach because I'd rather not have the overstretched
rubber hose rupture and leave me stranded on the side of the road.
If you're going to go to a conical filter, or something else slightly
shorter than the stock airbox (i.e. something that will allow you to
place the AFM a couple inches further forward in the engine bay), you
can avoid the whole stretching issue by finding a fuel-injected
Jaguar XJ6 in a junkyard and taking the short section of intake hose
on its AFM (I forget which side) which will fit over the outlet of
the BMW AFM (I think the Jag AFM is the same, so you might as well
buy it, too if you don't already have one from a BMW--the Jag's oil
vapor recovery system works very well, by the way, so an AFM sourced
from a Jag will be less likely to have the huge quantity of crap that
you find in AFM's on old Alfas and BMWs), and also fit OVER the stock
Alfa intake hose. This would probably work best if you but some kind
of metal ring inside the intake hose so it would tend to expand, and
seal completely with the hose from the Jag fitting over it.