8.5 good but not alot more power.
The helium does not have the power of a sail 2 metres bigger no way.
I have a helium 8.5 , and a maui sails 9.5 the maui sails has alot more power then the 8.5 especailly in lighter winds
Now the 8.5 is no slouch, and as the wind picks the difference becomes less and less.
BUT in light airs the 9.5 outpowers it .
I know as I raced longboards this summer, and I used my 8.5 till i broke a cam.
I then rigged the 9.5 and was passing people that were previously passing me when i had the 8.5 on.
simple as that.