Interesting. RNP is very precise horizontally, but with un-comp baro vnav, there is quite a bit of vertical leeway with temperatures.
While the criteria (FAA) allows the 3 degree GPA to only go to 2.71 GPA, the custom criteria has been driven down to 2.5 GPA. This gets you quite a bit more access with lower temps, but with the understanding that the terrain/obstacles have been surveyed to this level.
In regards to your Honeywell, or for that matter, any of the manufacturers prox systems, there last few years have evolved. By this I mean the electronics capability, data storage and the software.
Unfortunately, I have noted many discontinuities in the transitions. I look at this much the same as any other software, you have to keep bringing the old calculations forward, for compatibility issues. This is very much like being able MSFT Word being able to read a 2001 version document, and convert it to 2013 and then back.
In aviation, this is more based on the expense and time of certification, allowing to add to a cert, rather than get a new one.
more on this later...