.....back,
Reading the specs on the transducer that you have in mind, I see no reason why it should not provide good performance. However, as you likely know, with speakers and their components, specs don't mean a whole lot - the proof of performance is in the listening.
A few years ago, I used to own a pair of ESS loudspeakers which employed a similar midrange-tweeter driver called the Heil Air-Motion Transformer. It looked similar to the transducer pictured above, and it was based on a similar design, namely, a thin film-type diaphram behind a metal plate assembly. It sounded quite good.
As far as the array of the front panel, yes, it's called the D'Appolito array. One thing you may want to keep in mind is that, while such an array has advantages, there is a need to have the tweeter level at about ear-height (while sitting, of course) so that the vertical soundstage is accurately reproduced. I am not sure if you are building floor-standing or stand-mounted speakers as your mains.
Of course, as I am sure you realize, building a good speaker requires consideration of the interaction between the drivers, the cabinet, and the crossover network. Even manufacturers equipped with computer-aided design facilities find it a challenge to work all those considerations into a good-sounding speaker design. In answer to your question about whether or not you should invest in that transducer and use it, it's really hard to tell from here. Building a system should provide you with quite a few hours of satisfying and challenging work, and hopefully, a good-sounding system. In time, perhaps you can e-mail me and let me know how it turns out.
Oh, btw, I have owned Onkyo amps and receivers since I was a teenager. They make good-quality stuff at a reasonable price. Good choice.