The high drive speed reduces the seek time, since it doesn't takes as long for the disk to rotate into the correct position to begin read/writing. However, the effect is pretty small, since the "rotational latency" of the platter is only about a third of the seek time for the drive heads.
A higher speed also increases the data transfer rate, since the head is physically moving over more of the disk in less time, but you get the same benefit from increased data density without the disproportionate increase in noise, heat, and failure rates.
If you're still thinking about it, I suggest you instead get three non-Raptor drives and run them in RAID 5. The cost is similar, but you'd get better speed, reliability and capacity.