I now own a kevlar evo, carbon legend, and a vintage kev/carb red7 surf70.
Regarding Flatwater speed:
Since Think Boats hasn't responded to this thread I'll quote what designer Daryl R. told me two years ago, that the Legend has about 3% cruising speed over the Evo:
"… so at 10 kph in the Evo, your at about 10.3 in the Legend."
Regarding Stability:
I compared the Carbon Legend and V10S (don't recall the layup) in almost-flat conditions at Deep Cove in BC (a little power boat wake and breeze). The V10S wide seat and low initial stability surprised me with how tippy it made me feel, and I voted for the Legend there.
But later in Hawaii (thanks to Zsolt) I paddled a V10S in 3-4 ft directional swell and wind chop, sideways, upwind and downwind, and I thought it was great due to its high secondary stability. It felt a bit more comfortable there, than with my Evo in milder conditions on the Pacific Coast of Washington (at La Push). But of course there's the 30F water temp difference to mess with your head …
My opinion is in real surfski conditions, an intermediate paddler is far better off racing with a V10S or Evo, compared to a V10 or Legend. But I think for training it is great experience to get in a tippier boat on a regular basis. The junior team kids in our local flatwater sprint club are always switching back and forth between boats and seat heights. Problem is, you gotta have the boats to do it!