It's not usually so simple. Modern physical disks are intelligent devices, and giving them more parallel requests can allow them to optimize the head movement and lead to more global throughput at the expense of response time for an individual thread. Storage arrays and RAID levels can make a huge difference here, even if it is a single filesystem.
That said, rajaman didn't give any information on the hardware he is using, so anything we say about how to maximize his I/O throughput is just
PS: My issue isn't with whether or not parallelism will help with this particular problem, but rather the generalization that I/O bound processes can't generally benefit from parallelism. Storage manufacturers, OS developers and Systems Administrators put a lot of effort into making storage work better for different workloads, so you can sometimes be surprised by what your storage can do if you put in a little effort.