A few points to consider...
1. disk access is highly configurable at the time of setup (raid arrays and volumes) things like disk alignment, large clusters, and large sector sizes are contribute to better performance for large contiguous file creation aka D2D. I bet you could get another 10-20% improvement if you tune your disks properly.
2. 11d has some known issues with disk fragmentation that were fixed in later releases of BE12.x Try a good disk defrag app as well as playing with pre-allocated bkf files of a certain size.
BTW, LTO4 needs something like 80MB/sec sustained throughput to keep the buffers filled and the tape head writing. 9/10 no one is able with BE to maintain that for LAN based backups, thus D2D2T is the only option, or SSO and each client is a media server.