"Packet headers and control packets" are not the reason you should divide by 10. The reason is that Thunderbolt uses 8b-10b encoding so 1 byte of data is transferred as 10 physical bits. Therefore the maximum theoretical usable bandwidth is 20e9/10/1024/1024 = 1907 MiB/sec. Then on top of that you have to deduct the overhead from packet headers and control packets so the real-world usable bandwidth is even less than 1970 MiB/sec...