I believe touch screens are fundamentally a bad interface for productivity. Consider the range of actions provided by a mouse: You can hover without clicking, you can left click, or you can right click, all with nearly pixel-level precision. Add in a keyboard and your options expand even further.
A smartphone is like a computer with a one-button mouse and an abnormally large, irregularly shaped cursor, where can never be sure which part of the cursor indicates your actual click target. Software on this computer is not aware of the cursor's location until after the mouse has been clicked, and portions of the screen are blacked out when you move the mouse to certain positions. Your keyboard only works when you bring up an on-screen overlay which takes up ~35% of your screen real-estate, on a monitor which is abnormally small to begin with.
Could any amount of well-designed software make text entry efficient on this machine?
This is a hardware problem, not a software problem.