NIC Teaming Slow Throughput, any ideas?

I know this is an old thread but w/ a brand new Dell R430, TL2100, either single 1G NIC or LACP teamed (4) 1G NIC's, exact same experience.  ~300-350Mbp throughput overall, either B2D or B2T, no difference.  We're unable to get it above this speed, backing up VM's w/ the VMware Agent backing up VMDK's (not local windows agents).  Fresh install of Backup Exec 15 w/ all service packs, etc.  Equates to 2.5-2.6GBpm average, same as you.  One "large" 450GB VM does bump that up to 500Mbps throughput, or about 3.5GBpm.  Still unusable if you need to backup 13.7TB of critical data in 29 hours, let alone our overall 30TB of data.  Symantec's overseas support who are clearly just reading from a scripted checklist was a dead end after concluding that our 2.5GBpm/300Mbps was "already better than average."  We've engaged our VAR about what they sold us.