With OpenSSH agent support in the new RC, I had issues getting WinSCP to use a specific key. I was able to provide the 1Password-provided key in the private key field as a public key but 1Password had already had this configured within the .ssh/config:
Without defining the public key in the private key box in SSH > Authentication, at least 17 keys were attempted. If, on the other hand, I were to use Windows-shipped SFTP command it obeyed .ssh/config perfectly.
# 1Password Item: <item name>
Match Host <host> User <user>
IdentitiesOnly yes
IdentityFile ~/.ssh/1Password/<pubkey>
Without defining the public key in the private key box in SSH > Authentication, at least 17 keys were attempted. If, on the other hand, I were to use Windows-shipped SFTP command it obeyed .ssh/config perfectly.