Is it better to leave the stock air diverter on, or take it off? I hear the aftermarket ones make a difference, but should I leave the stock one on or just take it off? What will be better for the intercooler?

 

Unless you think you know more than the engineers at BMW, I'd leave it on.
AFAIK it allows the scoop to seal over the intercooler, and direct air *through* it rather than over it. If you really wanted to test it you could try repeated runs with/without and check AIT, but IMO after a decade if there were an advantage to removing it, miniacs would know.

 

i would leave it on, until you get an aftermarket one, air needs to be able to flow into instead of over it

 

Are you planning on getting a new aftermarket one?

 

+1
Keep it on till you buy a better one...even then their "better" is mostly for looks....