Could I successfully build out an opnsense firewall as a KVM, back up the config (using opnsense's built-in backup feature), install opnsense on physical hardware and then restore the backup to the new physical box? I don't see why it wouldn't work, but I haven't used opnsense before so I don't know if I'm expecting too much.
I dont know if the settings configuration of the manually installed packages gets saved. Can you try and see?
I think the default configuration you can apply anywhere
Your interfaces will likely have different names. You'll need to set them correctly from the console after you've restored the config.
Bart...
New to opnsense but expect this will be the same as pfsense. As Bart said your interfaces could change this can be because you are going from physical nics to a vlan setup or the virtual nic just loads in different orders.
If you were heading down this track I would take a good back and then try your new setup. Have a look at the how the interfaces are setup with their names and then in a text editor take a copy of the backup and in this substitute the old names with the new, load and test.
Next check that the interfaces are still using the correct virtual nics and are going to the correct networks as their order might have changed, easy to check using the Mac addresses.