Warning!
The data listed in this post is at, or above, the maximum published data. Do not consider my loads to be a short cut to a proper, and safe, load developement process.
The 7 mag. has been one of my pet cartridges for a long time and some of your observations are consistent with mine. I don't load with 780, I use RL22 instead, but still, I have much in common with you in regard to the results. I've been running 139 gr. - 145 gr. standard production bullets most of the time, Sierra's, Hornady, Speer, and Nosler and have found that they will all produce very good groups when my powder charge is up near or above max if the bullets are seated to / into the lands, using CCI-250's, and mixed head stamps necked, and, or, partial FL ( shoulder's maintained ) sized.
Any more I don't start my load developement below mid data range, it's just a waste of good powder and bullets. I wasn't surprised last year when I was working up a load for a 130 gr. Speer BT and started the RL22 load at 70.0 grains. I was consistenly shooting sub MOA groups right from the get go.
Your post has made me want to give that 780 a try. Good stuff you posted here, and thanks!