Most of all the 67-69 platforms came with an 8.2' 10 bolt. 8.5 are not all that plentiful for the narrower F-body. There are very few 8.5's that will fit without modifications.
If I were to compare a Ford 8/9" to a GM 10 or 12 bolt, I would agree. The reason for this, is that the pinion is so much farther off ring gear center line....and that has basic mechanical theroy and application to back it up.
This is the only real change that would add/subtract a measureable amount of 'frictional' loss and its not really a frictional loss its mechanical, (being of a compound/hypoid type gear).
As for a 10 or 12 bolt, the ring and pinion's are on the same centers and are only a 1/4" different in diameter, not to mention that the surface contact aera on the two are almost the same. As for weight, the 10 bolt is a little lighter but only by a few pounds. So its not that much of a factor...