Two quick thoughts about this:
- Ensure the SATA disks do not only meet the space requirements but also the provide enough IOPS
- If you are going to completely evacuate the VMFS3 datastore, then delete it and recreate it with VMFS5 rather than to upgrade it. This way it will be created with the new unified 1MB block size and the full benefits of VMFS5 (see http://blogs.vmware.com/vsphere/2011/07/new-vsphere-50-storage-features-part-1-vmfs-5.html)
André