Come on. Jumping in lulls and going up wind in 10 knots on an 8 meter kite and 128 board? This is a challenge to believe. It sounds like you were sold on the Sonic 8 before you set foot on it so rippin past someone on a 15 meter kite was destined to be your reality before you left the beach. I did not consider your review realistic.
I'll just clarify the ambiguity of my late night posting. I was managing to just stay upwind in the 10 knot lulls, (working the kite quite vigorously which is easy with the light steering), which were fortunately not too frequent. The jumping was closer to 15knots.
I'm only 65kg which helps a lot and the 128 board is quite flat rockered and goes upwing well. The 8m jumps surprisingly well in low wind if you can build up speed and momentum and have a good understanding of how to get the best out of the kite. I've been using a Sonic 11m since the first shipment in September last year and have learnt how to trim and fly these kites to squeeze out the best performance.
The guy on the 15m Tribal was approx 90kg and although an experienced kitesurfer had not had the Tribals very long.
The point I was making was that if the wind was light I was able to get by and have fun even though the 11m would have been more suitable. The range of these new bow/SLE style kites is much better than the C type especially in such areas as keeping the power through turns, and when you increase the apparent wind through speed and sining the kite.
However try one for yourself and see what you think. Make sure to give it time and it's essential not to choke the kite with too much backline tension. Keep it moving fast for the best performance unless well powered.
Have fun, Dave (Dr Surf Australia)