Think about this, folks-- to read or write a big file, it would take less I/O operations to complete at 64k than 4k. To get the same amount of data through for your test, less I/O's were required for 64k. And you may be running up against a bottleneck in your storage transfer technology (fiber channel?), being that 455 MB/s = 3.6 gigabits/sec, a healthy real-world number.
I'd say in your case, look favorably upon your 64k result.