Tape Drives ( LTO4) per HBA

Hello,

We are getting very low throughput ( 20-35mb/sec ) on current LTO4 Drives, i am sure we have to tweak some perf settings but whats the recommended/best practise number of LTO4 ( 4gb ) drives we can attach per 4gb hba port ( we have dual port HBA ). current configuration is:

HBA - 4gb  , LTO4 - 4gb & Switch port - 4gb sfp

Current configuration:

HBA1 Port a - 5 Drives ( Fabric A )
HBA2 Port a - 3 Drives ( Fabric A )

HBA1 Port b - 3 Drives ( Fabric B )
HBA2 Port b - 5 Drives ( Fabric B )

Appreciated any input.

thanks,

Raj