Downside of using a single DMZ vlan intead of typical internal/external VLANs?

I can't think of any major problems, but my 2 cents would be:

1. Like you said, 1 armed is half the throughput
2. You will be using your address space faster since real and virtual servers are in the same subnet
3. Helps with potential firewall mistakes. Say you retire a virtual server that has a firewall rule allowing port 443. You forget to remove the fw rule, and put a server on that same address, now you are allowing 443 direct to that server.
4. In the case of Internet and public addressing, you can private address your internal DMZ and public address your external F5 DMZ.