The limit on electrical power was originally set, as I once read a long time ago, because operators who used to do manual switching were at a higher risk of hearing damage and perhaps other problems when patching the lines. I don't know if it's true, but, that was one reason I read. I couldn't find anything to back that up in my search, so it's probably an 'ol techies tale.
I was able to read that in many cases you wouldn't get 56K simply due to line noise and other factors. As it stands, I barely ever get about 48K connections from anywhere to any provider's number. 53K is a dream connection. So, it's quite possible we'd get a few more K/s rates if the power was bosted, but, the chance of getting a 56K connect is pretty slim.