Wow forgot about that one lol, Yes an 8 or 9 will be right imho. My buddy runs a well tuned Rotax 800 Rhino. The one in that video racing Sandchris. Chris was running a 7 paddle I believe and Sancho with a 9 paddle. The 7 paddle won this race easily.
One weekend we had all the different rims and paddle set ups. Sancho tried them all, and decided the 9 were the best all around on his Rotax.
I guess you get the grab and keep the top end also.
I tried a few a while back really didn't get to make my final decision. I will be running 14 inch FST Rims with 8 paddles in the rear and buffs up front.
I will probably end up with a 12 inch rim and paddle set up with 9 paddle rear and the new buff razor edge tires. This decision has alot to do with availability