"IDE" is a very generic term these days. What specific version/speed are you using? What hard disk are you using? What's your typical access pattern look like? What's your bottleneck?
A typical hard disk today might be a metal platter on SATA 2 (3 Gbps on the wire). The next SCSI version faster than this is Ultra640. I would say that SCSI drives tend to be rarer and more expensive and lower in capacity (so 10 TB would probably take a lot of drives!), but I don't think anybody makes a metal platter on Ultra640 at all.
A fast metal platter hard disk might be in the neighborhood of 0.5 Gbps for sequential reads, so even SATA 2 is many times faster than it. Seek time and latency could be a little bit lower for SCSI drives, but that depends entirely on the particular drive. If you're read-heavy, mirroring your existing drive could help with latency. If you're concerned about the hard disk's cache being faster than the interface wire speed, adding RAM would be a much easier and cheaper solution.
Again, you've not told us what your bottleneck is, so we can only guess, but it would have to be a pretty specific set of circumstances for the hard disk's wire protocol to make any difference at all.
Most of the very fastest drives available today are SSDs, which nobody seems to make in SCSI any more, and certainly not at any wire speeds greater than SATA 2. The fastest storage devices are solid state but go straight through PCI-Express. And companies like Google seem pretty concerned with performance but don't bother with expensive parts like SCSI.