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.
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.