From everything I've read, the UCAC 2/3/4 catalogs appear to be much more accurate in terms of star positions and magnitudes compared to the currently used Hubble Guide Star Catalog. It would seem to make sense to use UCAC down to a certain mag. and potentially offer a much larger portion available as an additional download for users who want to go even further and don't mind the added GBs to do it. Another astronomy software I use has the entire UCAC3 database and it comes out to be about 4.7 GB total. Over 12 GB with UCAC4, but that's overkill for most people. Starry Night 7 does something similar, where many stars are included in the installation and more can be downloaded separately for those with the need, so SC has done something like this before.
Update to the UCAC Catalog for SS6