If I have it right then you suggest a bunch of disks each in a RAID 1 mirror, then a RAID 6 spanning the mirrored disks.

I can see there is greater protection than RAID 10, or 15, but what is the actual difference in risk between RAID 15 and RAID 16?

For total failure:
RAID 15 requires a minimum of four failures in less than the time required for two consecutive mirror rebuilds.
RAID 16 requires a minimum of six failures in less than the time required for three consecutive mirror rebuilds.

I haven't seen a RAID 1 rebuild happen in production, I don't have stats on the failure rate fo mirror rebuilds. I have never heard of a RAID 1 rebuild failing, but I am junior.

RAID 16 requires a minimum of 8 disks.
Probability of losing both disks from one of the mirrors from the overlying RAID 6 are 1/7th of the probability of a mirror rebuild failure, or lower.
Now you have same resilience as RAID 15 (or near enough).
This is the advantage of RAID 16 over RAID 15.

Given the conditional probability for total failure of RAID 15 what risk are we looking at here?
Is the extra layer fo RAID 16 really adding protection that you need? High Availability is HA, but what are you aiming for that you need RAID 16 rather than RAID 15?

Can anyone give an expected failure rate for RAID 15 or RAID 16?