As the others have said. To get the double performance from each drive you have to partition them into two 7tb drives each.
5 phy drives would be seen as 10 seperate drives of 7tb (pre-format).
Then tell DSM to stripe the drives so data is spread over all 10 drives.
Hope that makes sense.
Basically these are two in one hdd’s and you need to partition them as such so the OS uses it that way.
Wendell has a decent video here explaining these drives.