Full disclosure: I've only ridden 59cm raceboards, not 70cm. Also I've never ridden an edge, I ride a Dyno 18m. But I will add my 2 cents anyway
My favorite thing about the raceboard is getting out when the wind is super light and the water is mirror flat. The big kite lets you go crazy fast in these conditions.
I know everyone measures and reports wind differently, but my low end with 59cm raceboard and 18m kite at 165 lbs is 7-8 knots. If you really want to get out in (my interpretation of) 8 knots, then you'll need at least the 15m.
With the raceboard, you're not working the kite much unless you're going straight downwind, so a smaller kite isn't more fun than a larger one. Racers typically don't move the bar at all, even for trimming in and out they use the trim strap.
I can see how on a more maneuverable board like a sector though, the 13m would be more fun to loop through fast carving jibes.