>I usually multiple bytes * 10 to account for overhead
I don't disagree with your point, but this bit is faulty. If you want to account for some overhead you should multiply by a number lower than 8, not higher.
No, he's right here--just looking at it from a different perspective. He's saying "it takes roughly 3.5Gbit/s of bandwidth to support 350MB/s of real data transfer."