Slow Exchange DAG SAN Media server backup

NBU 7506 (Windows)

Exchange 2010 4 node DAG with NetApp as iSCSI storage

Tape drives are zoned to exchange servers

Seperate policies are created for respective server with DB mounted

local disk backups are 100mb/s+ ... whereas Exchange SAN backups don't exceed more than 10mb/s... its strange!!!

SAN topology is of 4 gbps

buffer tuning helped local backups to offer max speed... but don't know what could i tune here... Can Rep Director help here?

Also wanted to know how Snapshot policy can help reduce window?