We're using SCVMM and Hyper-V hosts to manage our Automated VM environments. Currently we have two servers with 3 7.2k consumer drives each (one for host OS/Storage, two for VMs). It's hard to say if it's the quality of the drives, but right now we can't get more than two VMs to run concurrently on each drive without it interfering with automated UI testing. I say it's the drives because we have excess memory availability and low CPU usage.
I was looking into some input as to the benefits of SAS vs SSD vs SATA drives for a situation such as this, since we're adding a third server and it's a good time to rework our storage system. The actual automation has fairly low IO demand but since we're attempting to run as many machines per drive as possible, I'm not sure what to go with.
Research has not been terribly helpful as most articles and forum posts regarding this subject revolve around home server use or virtualized server/database use.