Thanks @opichals I'd come to that conclusion re the SDK.
Re use-case - honestly? It started out as a learning exercise, but I've found it to be very useful and reliable. Re performance, I don't have any benchmarks, my apps are sending at 50ms intervals with no problems, but I appreciate that's not particularly fast when compared to MQTT QoS 0 which I think is the best comparison. That's not to say the interval can't be much smaller. I haven't tried.
I could run Mosquitto locally, and may end up doing, but this has fewer moving parts to achieve (or at least emulate) the same thing. That was the attraction really.
I'll look into CoAP though since I've no experience with that. Thanks again.