ok to clear all of this up
i am assuming you have the new maps modded on you HD
those will be the only ones you can mod on xbl
if they are on your hd even after you turn your chip off they will still be there
yes you will get banned from MATCHMAKING not halo 2 online entirely you will still be able to play customs
i personally mod halo 2 on xbl but its in customs i have a second factory default xbox to play legit
so if ypu just wan to have fun with your friends i see not problem with that