Since I loaded the new 15.1/5.1 hybrid without removing power to the DVD (thus leaving the 5.1 KWI in non-volatile memory) I don't yet know if this new combination will survive a full firmware load if/when battery power or fuse is removed and needs to reload. What I do know is I don't want to load the clean 15.1 DVD as that for sure will update the firmware and remove the bypass/override capability.
For the record I also created a hybrid DVD with the 14.1/U39 map data and that too worked fine with the 5.1 loading file.
I do find it interesting that there doesn't appear to be anywhere in the Nav UI that indicates that this is the "West" version of the 15.1/U40 DVD. The only difference between West and East in the U40 data are the POIs since all map data is on both....but it would be good to know which one you had loaded at any given time.