> Using sixty threads to write 2GB files at once using 64KB writes, results in 1099MBytes/s at 0.06ms service time.

It's not a good idea to scale disk writes using threads when you can use aio. You've introduced unneeded scheduling and locking penalty from threads. Additionally, such threads will be accessing and modifying kernel structures frequently and that's going to cause additional locking inside the kernel. They'll all be modifying same resources and structures while writing.