I love my API on snare, but I've never preferred it on kick - I love big/deep kicks and the API feels a bit thinner and more mid-forward to me than some of my other pres. The API does really well with transients though, so if you're looking for punch it'd be the way to go.
I'm a big fan of the Dual Mono as well, it feel a bit thicker/weightier to my ears than my API. I'd probably use the API on snare and the Dual Mono on kick.
I haven't used the 215 but I do have a Focusrite ISA428. I'm not sure how different they sound from each other, but the ISA428 is a good all-around pre and it generally gets the job done on drums. That said, I wouldn't generally pick it over the Manley or the API.
I'd do API on snare and ISA on kick, or ISA on both. Manley is great, but it's soft and round and is better IME on things like vocals or bass, or even putting on something after it's already recorded for its richer color. I haven't compared the 215 to a 428 in a direct head to head, but I remember them being on a different level of quality, with the 215 being a big step up.
Thanks for the insights gents! Subspace your setup resonates the most with me. I prefer the Manleys on OH as they're a little softer and will smoothen the cymbals with the faster 215 capturing the transients of the kick and the 312 capturing the balls of the snare....