Hard drive speed and quantity is only a small piece of the disk performance picture. You also need to look at your workload characteristics, caching configuration, disk alignment, RAID array type (1+0 vs. 5/6), segment size/stripe width, and whether you're inadvertently mixing random/sequential workloads (or multiple sequential workloads) on the same set of spindles in such a way that it drags your application performance. If you can't get these basics down on local disk, a SAN is unlikely to help you besides providing you with the ability to add more spindles (disks) to service your I/O requests.
A SAN represents a significant initial cash outlay, a significant yearly support/maintenance fee, a significant amount of supporting infrastructure (iSCSI/Fibre Channel), and a significant amount of learning to support correctly. Unless you're really sure you need one, you should probably exhaust all the other ways of improving your application's performance first.