1) Not necessarily.
2)Performance degradation could be various things.
3) RAID 1 is generally slow, but that is extremely slow.
4) As far as boot partition, that really only holds true with older boxes, I do not believe newer servers have that issue, especially since that has more to do with file system than anything else. Also, it's a non-issue unless you plug in a 3TB or 4TB drive to begin with.
5) Transferring disk image arrays can get dicey.
6) BIOS or UEFI doesn't really make a difference honestly. UEFI is more user friendly.