When you set throttling, it will use only the specified bandwidth no matter how many streams you have. I've got my aux copy set to 20 streams and when sending data to our offsite disk array, our network guys confirmed that the limit was respected.
If it's not the case for you, you might want to review your settings and/or open a ticket.
Where did you set your throttling rule? Are you copying from a single MA to a single MA? What service pack are you on?