Can you tell me more about how the on-premise installation works and/or is licensed?
Can it manage my authentication mechanisms for me? For example, can I configure it with my client side certificates or have it fetch and cache oauth tokens? We do this in our current solution and it is very nice being able to hide all these details from our applications.
Can it do request/response transformation at all? We have a lot of cases where we want to massage things a little here and there. I realize this might be out of the scope of what you are trying to do, but it would be a nice to have.
We currently do this sort of stuff with a cluster of IBM Datapower gateways. They perform very well but are expensive, difficult to configure, and somewhat opaque.