If you do manage to split this into threads you may actualy reduce performance as each time a different thread gets a shot at it it forces the HD to drag it's heads over to a completely different part of disk. A single thread reading the file sequentialy will not be making the heads seak so much, assuming the file is not desperately fragmented on the media.

That may be true if the file is stored on a single disk. But somehow I doubt an 8-way box dealing with 45-50Gb files uses filesystems that are layed out over single disks. It's far more likely some kind of volume manager (either in software using Solstice Disksuite or Veritas Volumemanager, to name two common products used with Solaris, or in hardware, either by using a RAID card, or by having the RAID done by the backend storage, which in turn, could be done by a specific diskarray, or by the network itself (NAS)). Without knowing more about the volume layout and implementation, it's hard to say how much performance is helped by using separate threads or processes. It becomes unlikely that performance will actually decrease, although bad volume setups happen all the time. Often unknowingly, but also because people want to know the disk a certain file is stored on.