My assumption on the efficiency part has always been not that it is a limit on how fast you can take the power, but how much power it takes to stretch the body farther. Like a rubber band, stretching it to double its size takes a certain amount of effort, but stretching it from double to triple takes more than double that effort. The further that you stretch from original the faster it uses the power. So in theory, if you were pulling from one metalmind, and dumping into another simultaneously, the power would just flow from one to the other and since it is not stretching the body remain mostly unchanged.
That said, that is where my brain is on the situation, I have not looked through WoB to find confirmation, just seems how it is described to me.
