People here may make fun of it but ...
I think the BFR in 45/70 is a great idea.
Why?
Try finding a repeating rifle chambered in .454 or .500 Linebaugh ... they should make them, but for whatever reason no one does.
If I suddenly felt the need for a handgun/rifle combo in the same cartridge, say for defense from bears or something, I would choose the BFR and get a Marlin 1895.
That aside, Shooting Times did an article not too long ago on the BFR revolvers and included details on acccuracy and velocity. Don't hold me to it, but I'm thinking that the velocities in that 7.5 incher were pretty 44 magnum-ish, say 1300 - 1350 fps for a fairly heavy bullet, maybe 405 grains.
I think accuracy was about 3 inches at 50 yds., and they felt that these were definitely high-quality, sturdy guns..
I've wanted one for a long time, but with a dealer price just under $800, its pretty low on my list.