Designing my own boat (surf ski) using KF was a fantastic learning experience. I would reccomend any builder to have a go at their own design even if they don't get around to building it. I learnt so much about boat geometry.

I have an Epic V10 (6.5 Metres) & a Red7Surf70 (6.7 Metres). They get along quickly but its downwind paddling where the length is needed. Paddling into the wind is good for a workout but running with the ocean and wind is what its all about.
Check out link for some excellent downwind footage
.http://www.surfski.info/index.php?option=com_content&view=article&id=971:clint-robinson-video-updated&catid=86:ausnz&Itemid=166
Can't see any reason why a long kayak like Bjorns "sea racer" or Nick's "mystery" couldn't do the same but as mentioned by Bill and Paul, the long skinny craft need a decent engine. Not much good for anything but slamming it hard.
Cheers