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.
Oh, I dunno. Remember that repeat letters with QWERTY are distance-less, while with 8pen they always cost the same (just look at how to type "pizza", as they have it at the end. Even assuming best-case, that's hugely more costly with 8pen).
There are also few advantages / disadvantages with letter combinations, and I'd bet most QWERTY words average close to 1/2 the keyboard-size travel between letters or less. That's slightly less than 1/2 the narrow width of the screen, in straight lines. A single, near-center gesture seems it would exceed that pretty quickly, especially as people are likely to be much sloppier than staying near the center.
Sure, loops are fast, but are they as fast as a move and tap? And how about ergonomics - is it more tiring, given that the distance travelled is huge compared to many others?
Totally-blind typing: awesome. Huge advantage there where accuracy is important (Graffiti ain't bad, but small errors can magnify as you may cause a command gesture if you aren't watching). I'd call it a definitive "win" there. But how about other uses?