Performance problems with external SAS LTO-5 drive

Question

I recently got an external SAS LTO-5 tape drive from eBay for my NAS at home, and a SAS PCI-E adapter - this is the sas2flash output:

LSI Corporation SAS2 Flash Utility
Version 20.00.00.00 (2014.09.18) 
Copyright (c) 2008-2014 LSI Corporation. All rights reserved 

        Adapter Selected is a LSI SAS: SAS2008(B1)   

Num   Ctlr            FW Ver        NVDATA        x86-BIOS         PCI Addr
----------------------------------------------------------------------------

0  SAS2008(B1)     20.00.07.00    14.01.00.06    07.01.00.00     00:01:00:00

        Finished Processing Commands Successfully.
        Exiting SAS2Flash.

I had to flash a new firmware on the card (it had a "fault state" message on dmesg, but now it's fixed) and had to patch the LTFS userspace tools to get them to recognize this model, since it's made by IBM but a Dell rebrand. The patch is here.

All is good, but I'm getting poor performance from it. Usually, reads from my RAID5 array are in the 70-110MB/s range over Samba to the rest of my home network, but I can only get the tape drive connected to it via the SAS port on the adapter card to do ~22 MB/s, which makes filling up a 1.5TB tape a lengthy ordeal.

Is there any way to debug where the problem may lie? The NAS is on a mini-ITX motherboard and rocks 8GB of ram and a 4x3TB RAID5 array, plus a boot SSD connected to one of the adapter card's internal SATA ports. The SFF-8088 cable is 1.5m long, but the drive just currently stands on top of the NAS case.

Any help appreciated!