I feel like there's a lot more that we need to do before we can conclude anything.
Let's start with the connections. Have you tried other SFP+ modules? Other fibers?
Now, the other problem you might have here is either a power issue that manifests when installed a jail, or a data transfer issue when you're pulling down the jail. Let's rule either of those out: set up a basic share and hammer the bajeezus out of it. Write a ton of data, and read a ton of data. Does the connection go down? For good measure, start a scrub at the same time to really push the power envelope.
Now, let's rule out the 10G module. Try connecting directly via copper and doing the same thing. Any dropouts?