EDIT: As I'm being fairly specific about particular models here, perhaps I should clarify that I am a local JVC dealer, along with other products, readers should take this into account.
My impression was that apart from the color saturation that I described above, I wouldn't have been able to tell if it was an HD100 or an HD750 that was running, without a side-by-side demo. It basically looked like what I've been used to from HD100's. I wasn't impressed by the light output, but then I have no experience with the screen that was used (Stumpfl). Probably a fairly low-gain screen. Blacks were impressive, but so are the HD100's blacks. I believe it was set up with either a little too high gamma, or a little too low brightness (without knowing the source material, it's hard to say which). Not the best picture I've seen from JVC's, but I'll blame the setup, not the hardware.
The 4K demo was quite impressive however. It was quite interesting, though not surprising, to see that the JVC logo in the beginning of the demo looked oversaturated on the HD750 that is said to have DCI primaries, looked fine on the 4K that also has DCI primaries - but was obviously fed by a DCI source. A very nice demo of why the primaries should match the source, and that you can't just convert a normal HD source to DCI in the projector and expect proper colors. With the DCI source, the colors of the 4k PJ looked spot-on, and the contrast was noticably improved over the 4k demo they showed 2-3? years ago. The limitation of this demo was clearly in the cameras used.
Basically, I'm sure that the HD750 post calibration will be a very noticable improvement from the HD100/RS2. Pre calibration I'd say they are very similar. Lumagen Radiance owners shouldn't have much need to upgrade their RS2's... ;-)