As much as Cain has improved in these 7 months, it's highly unlike that it's enough to have her break 18:00, much less 17:20. You do realize that, right?
No, as a matter of fact, I don't. If you don't think that someone who can run in the 4:30's for a mile is incapable of a 17:20 5K, even with a relatively steep performance curve, then you are deluded. You are clearly operating from a position of having decided your conclusion in advance (Cuffe is a better distance runner than Cain) and scrambling to come up with evidence to support this. This is not making you look very smart.