If you do your load research, you will be amazed at how much power can be obtained with trapdoor level loads. That is if you reload anyway. With the right powder, 300 grain bullet velocity can easily exceed 2000 fps within the 28,000 cup trapdoor limit.
I dont know what pressure the 45-70 LE is loaded to, but it is possible that it does not exceed the 28,000 cup limit. Again though, I have no idea. Traditional factory loads from the big 3 are usually loaded in the 18k to 21k range I believe.