This is measurable- the minimum diameter of a circle passing through the four quadrants is the maximum distance the thumb can ever have to travel. Allowing for some safe radius from the central dot, you would end up with a shorter travelled distance for many letters, even in the worst case. Add to that the fact that qwerty is not optimized at all for the positioning of the thumb- so G is a farther distance away than Q for example- and we're talking major benefits.