Years ago I read an article about spitzers in the 45-70. I think they were using Northern Precision bullets, but I can't be sure anymore. In any case they found that spitzer bullets did not flatten the trajectory of the 45-70 that much. This is due to the fact that bullet drop has a lot more to do with initial velocity (or more properly, time in flight) than B.C., at least at the 0-200 or 300 yard range.
However, at 150 yards and beyond the spitzer bullets were arriving on target with considerably more energy than flat point or roundnose bullets. Now, looking at Tim's ballistics chart, this bullet does make the 45-70 a nice 250 yard gun if you sight in for 200 yards and if you can stand shooting those pills at 2400fps. That is nearly 375 H&H velocity in a much lighter rifle. Start them off at the 1900fps of the 458 Socom and the drop should be much more significant.