I have no idea about the specifics but from what I've heard developers really liked the XBOX (despite it's unholy internals) so I'd say the API was actually nice.
As for HLE based on the API it's definitely possible but there's probably tons of nasty corner cases and that's my theory on why it's never been done. That and the chance of games with their own proprietary APIs existing and thus they won't work with that solution.
As for HLE based on the API it's definitely possible but there's probably tons of nasty corner cases and that's my theory on why it's never been done. That and the chance of games with their own proprietary APIs existing and thus they won't work with that solution.