Wow pressing the key harder for upper case is very good. I would take that in a regular keyboard.
But I'm not too sure how useful it is for efficiency even after practicing so much with it; there are a lot of false positives and negatives since keypress velocity doesn't matter for regular typing. With enough data, you could figure out a specific velocity threshold for each key so that you make fewer mistakes, and then maybe it'd be more helpful.
It can be made in a mechanical way, like a tristate button -- off, slighly-on, on.
I also suspect that the keyboard layout is more standardized between keyboards than the force threshold would be.
(And, it occurs to me that we're essentially chording with the shift key.)
Someone needs to make a velocity-sensitive mechanical qwerty keyboard :)