Throughput is fine. Latency is maybe 10x higher, but that's going from tens of microseconds to hundreds of microseconds. Not really a noticeable difference in most situations. With VT-D, you can hand a whole PCIe card directly to a VM. In that case, latency is still higher than on dedicated hardware, but less so. Virtualization costs a lot of I/O latency.
The larger concern is the failure domain. If your VM environment goes down (e.g., your datacenter loses power and all hosts need to come up from scratch), do you need that firewall working to be able to get tech support and/or recover? With virtualization, it's entirely possible to set up your environment in such a way that it's impossible to recover from a full outage.