The box definitely looks worth getting, especially if it's complete with respect to their studio sessions. I have only one of the aforementioned Nostalgia Arts issues (Vol. 2), and John R.T. Davies is credited for the remastering. That disc contains material ranging from 1925 to 1932.
An earlier (1994) CD reissue (Vol. 1) on another Carl Hallstrom label (Collector's Classics) contains material from 1931 and 1932, and JRTD is again credited with remastering. As far as I know, there never was a volume 2 in that series, and the relationship between Vol. 1 of the Collector's Classics cd and Vol. 1 of the Nostalgia Arts cd is unclear.
Note also that there's another disc that may be of interest on Retrieval (The Boswell Sister, 1930-1935, RTR 79009). It includes airshots that most likely will not be on the Storyville box.
Is there a listing yet of what the box will contain?
I believe that the late John R. T. Davies did the transfers for the original Nostalgia Arts issues. Of course, sound quality is highly subjective, but a good many collectors would still consider Davies' transfers of 78s the gold standard for reissues. I have bought and cherished many Mosaic sets, but I don't think it's clear that the label has surpassed the level of Davies' work.