Dave -
As far as my limited research on this subject went, I found no stereo shop even willing to consider replacing the head unit with anything but the stock unit. The systems integration makes the head unit a permanent fixture - at least, for now. *Hopefully* someone will make an aftermarket stereo for the Niro, but I am skeptical that anyone ever will.
Amp/speakers/sub seems to be the only real way to customize the stereo.
Now, it is possible that the HK system for the Niro could be installed, I didn't look any further into it when I discovered our cheaper head unit is a $1600 part.
Considering our warranties only cover the dash electronics for 3/36, I'll be selling mine in the beginning of year 3 latest. Probably much sooner, as the lack of bluetooth text support is a 100% deal breaker for me.
But PLEASE, if anyone here can contradict my reply, do so. I'd keep the car longer if I could put a head unit in which is better suited to my needs (and sounds better).