I think the gap between the 9 and 13 is perfect, my 9 and 11 are a bit to close.
It's true that the hydro is a bit slower to get to the window than high AR-kites, but not than the CO2 -the turning speed is fast enough to let you do whatever you like - It's my impression that the hydro needs less input than the CO2 to turn. A cabrinha kite takes some musclework to steer, but the CO2 i besides that a lot like the Hydro. I sail a lot with a very good wakeboarder (Morten Lander, 25th at red-bull skyride, Tarifa) who uses CO2 - the sizes of CO2 matches the sizes of the hydro, and they have the same caractaristics - upwind ability, low end etc..
The hydro is super-stable, but still a good performer. Last year I used Airblast, but i really think that hydro is a better kite for most people. It' so easy to control in even difficult conditions. That type of kite is what we've been waiting for. I use it in competition as well, and I have never regretted it. I've seen people learn to go upwind on their first kite-day with the hydro 9 sa well. This new concept is bringing a lot of joy into kitesurfing for many people, including myself.
It is even possibly to fly them very overpowered, as they will never jellyfish due to the low AR. Last thursday i didn't bother to swap from my size 11, even though that the second biggest kite on the water was a 8,4 and most people were using 7 or less...
ps, I' try to test the 13 as soon as possible, but we have got to much wind at the moment