I have not used one (but thanks for posting and bringing to our attention)
It does seem you are correct in your thinking as to how it's connected and also the Warning Lights. The Speedo function will need a calibration number to be programmed in by you and that can be fined tuned over time to be bang on (or how you'd like it), if it programs the way other Smith's electronic speedos do, it's easy, so don't sweat that. It can be ordered with a 'Mini Gearbox Adapter' to that's all fairly straight forward.
Just a passing personal comment really and one for you to think about and decide upon;- I would think that the digital readout for the speedo annoying to say the least and it does appear to be on the small side. I am sure that you'd find it slower to read your speed from as a result, but having spent that kind of money, and the total work needed to install, you'd possibly find it hard to move away from, if you were less than happy with it.
I like the idea though, but I tend to think that the Speedo taking the 'major' face and the Tacho a smaller one would be a better arrangement for the road. Given the overall diameter of the whole gauge is only 100 mm (4"), the speedo numbers at my best guess would only be about 6 mm (1/4").
Don't take my personal comments all as negative, those are just my own views / thoughts. I do like the idea of having all the gauges in one place and can de-clutter the dash as well as making space that can be better used for other things.