Ken,,wetted area and waterline length are rough approximations for comparing cruising efforts and top speed potentials for different boats,,not one to one determiners as you mention given the nature of the motor but also because those numbers are just that,,numbers, not the results of tank testing.
When you set into a kayak and paddle it YOU will decide what is your cruising speed and YOU will discover what YOUR potential speed is. BUT you will also discover that a short boat will begin to bury when paddled hard and a long boat will continue to increase in speed with effort,,until you taste blood and explode.
I bet the charts Vaclav/One Ocean or Pygmy uses to compare their respective boats are accurate enough to be worthwhile.
As far as looking at numbers for "a good work out" my sense is that will be a function of whether the boat fits you displacement wise and cockpit size. In other words,,there's more to look at. If you're "working out" on waves then you want a kayak with good wave handling,,If you're "working out" on flat water then maybe a skinnier kayak might make sense.