Yes, you can use RDMs with vSphere Essentials. RDMs may or may not be faster, and many consider this a wash. It would be worth testing for sure, but don't assume that you will get significantly better performance with one vs the other. RDMs can also complicate things like backup strategies.
Also be sure to check out: http://blogs.vmware.com/vsphere/2013/01/vsphere-5-1-vmdk-versus-rdm.html#more-6563