Your numbers do seem quite low. My environment is mixed, but here’s what comes closest to what you described –
Exchange 2003 on Win2K Server, an old HP NetServer with 2x1Ghz P3’s and 2 Gb RAM. Fiber (SX) Gb card connects to our production Ethernet network and backups happen at off-peak times through HP9000 server running HP Data Storage Protector with 2Gb/s connection to a SAN which hosts a dual-Ultrium drive library. Compared to other attached systems the Gb Ethernet is definitely the bottleneck for this server, but we still routinely see 20-22 GB (bytes) per hour, even with low user activity on Exchange.
Are you using jumbo frames on your copper Gb connector? Also, RAID 5 is really not the best choice for the kinds of sequential write activity in backup to disk. Can you try a simple RAID 0, or if fault tolerance is absolutely necessary, RAID 10?