I have two exchange servers (E1 and E2) where E1 is the older system and E2 has all the mailboxes moved over.  E1 backs up at a rate of 500MB/min on the IS.  E2 on the other hand backs up the IS at a rate of 50MB/min or less.

E2 is newer hardware and all 3 machines sit on the same gigabit switch.  E1 has a Broadcom Netxtreme 1 while E2 has a Broadcom Netxtreme 2. Both have drivers as recent as 07/2007.  Haven't seen any different settings on what is available through each nic.  Though, the backup machine has a 100Mbit network connection.  But, yet, at the same time, it backs up the IS on E1 at a much greater speed.

Don't see any collisions to E2.  A file/share backup seems to yield different results on E2 with a rate of 300-500MB/min.