It's not terrible, but you're probably not going to see all that much more than that. Let's do the math... what you're seeing is 360Mbit/sec. The currently available SATA standards (which is what's on the drives themselves) are 150Mbit/sec, 300Mbit/sec, and 600Mbit/sec. Assuming mechanical hard drives on both sides, I'm frankly amazed at the numbers you're getting and suspect that I'm doing the math wrong. You shouldn't be seeming much more than 20MB/sec, if I am doing the math right.

I am the Doctor, and THIS... is my SPOON!