1) The protocols can both before within 5% of each other for 99% of scenarios. Ask your storage vendor what they prefer. I'm gonna guess QNAP probably prefers NFS.
2) NFS and iSCSI can both do VAAI depending on your storage...QNAP doesn't support NFS VAAI...
3) Sure.
4) No way to predict - its primarily limited by the performance of your storage (which is primarily limited by the disk and RAID configuration)....what can your array do?
5) doesn't matter for performance.
How are you testing performance?