1 - Is there a hard stop at 50 VMs and 5 hosts or is that just the recommended max?
2 - Will going to vCenter Appliance 5.1 gain us anything?
No there isn't, in fact I didn't considered this limitation for a while and now I have something like 2 hosts and 750 virtual machines on a vCSA 5.1 with ProsgreSQL.
It's still everything ok, if you don't consider the long delays using the vmware api (~5 mins to start or suspend a vapp of 4 vms).