Yes, there are seemingly compelling arguments for a shorter boat. BUT you will never, ever - unless you are compulsively overconcerned with testing it - notice the difference between a boat that is 15', and one that is 17' long. Just build Tom's Sea Rider, as is, and you will be very pleased indeed. Really.
His Rolldarka is also a very excellent boat, and both of these will fit you just fine. The Sea Rider has slightly more volume.

If you really, really want a shorter waterline, I can send you the guide for the 1931 Disko Bay, which has the same length, but a much shorter waterline. I absolutely couldn't tell a difference when it came to paddling effort or speed, though. You can see it here (scroll down): http://gentrycustomboats.com/New%20Homebuilder%20plans.html

Good luck!
Dave