There are a lot of variables that affect VM performance. HT is just one of othe variables but it also depends on how the VM are configured.
One of the variables that affect HT depends on the type of Hypervisor being used.
If you are using lightweight virtual machines like VZ, VServer, etc, then it is quite obvious that HT would bring suitable benefits as these VM aren't actually tightly isolated and are more like running multi-threaded software.
If you are using heavyweight virtual machines like KVM, Xen HVM, etc, then it is possible that HT will interfere with things e.g. when a VM is allocated two CPUs but ends up using one HT each on different cores.
So, the best thing to do is still to test your setup to see what suits you