OK that error is interesting, that kind of indicates that it was going to work but didn't because of auth issues. I've fixed that exact auth issue before so I know what is going on.
It requests a download link for the object rather than trying to download it via ACD directly. You used to have to do this for objects over 10GB.
That means you will always use the temporary download link.
I think the kernel caps the max read ahead to 128k so setting it really big won't help. If it did work then you don't want to use 14G of memory so I don't recommend you set it that high!