Pretty much regardless of SAN vendor, far too many SAN fabrics are woefully inadequate in terms of overall link speed. Given that the average SATA drive these days is capable of 6 GBPs data transfers, but the high end fiber connections are just 10 GBPs, it's clear that without multiple paths to a SAN, I/O speed in a "disk busy" environment is going to suffer. I would at least see what your SAN fabric utilization stats look like before you look at the disk side of the house.