Off the top of my head the Thunderbolt 3 to Thunderbolt 2 is about £45 and the Thunderbolt 2 to Firewire 800 is about £25
I think the eSATA adaptor was about half that! Also, I wasn't sure how much use I was going to make of the RAID box so probably wouldn't have paid seventy odd quid.
Somewhere there's going to be a bottleneck no matter how fast my cables are. I think we're saying that's my RAID 5 write speed for copying and I bet my Mac for rendering large photo files.