I was banned permanently from the Forza forums for telling them that their banning practice was unfair.
Of course any alteration of any car beyond what was intended can lead to banning in Forza and of course it is just lame to take a hacked car into an online lobby unless of course the lobby is for hacked cars where others are doing the same thing.
I have not tried to hack any of the modern online games but in the past I wrote an editor for NFS Porsche Unleashed that allowed for pretty much any kind of mod you could think of and gave it to all my friends. We would get together and race our modded cars and it was a lot of fun, kept us interested in the game for at least another year.