That's right!
-7's and -9's have the same size in specs.
From memory when speaking to Racepace, the reason why the -9's have better top end is due to the configuration of the turbine blades.
Ones longer and the other shorter (the shorter blade allowing more air to flow through).
Also to do with the pitch angle of the turbine blades.
It's at a certain degree to allow more air.