The only one who can answer your question is -- you.
The speed in which you can paddle a canoe depends on the interaction
between your abilities (power, endurance and paddling skills) and
the actual behavior of the canoe (its resistances and tracking).
Add waves and wind, and the whole thing will get a lot more
complicated. If "efficient solo canoe for sit'n switch paddling"
were my goal, I would instead be looking for sleek designs like a
We-no-nah Advantage or Bell Magic, and plan at least 2-3 paddling
workouts a week, to be able to get this designs to realistically work
at high speeds... If not, you may be going at 5+ mph for an hour or
so, but on the long run you will be paddling much more at lower
speeds, where designs like the Prism or Voyager offer no real
advantage as the We-no-nah Advantage and Bell Magic may do,
especially the Bell Magic (IMO). My advice is to paddle your current
design untill you are very tired, and then right after that,
test-paddle the boats you are interested in... This will tell you
much more about the speeds of these designs, than all our
'intellectual' reasoning about it ;-)

Dirk Barends