i3, i5, i7, and i9 have really just come to mean Good, Better, Great, and Best within each respective line of Intel CPU. Usually while the i7 and i9 models have higher clock speeds, they are also better binned processors (meaning they can achieve those clock speeds without spending much energy over their i5 counterparts).
The notebook i9 is still an i9 relative to their notebook processors. It's still a 45W TDP processor, just like the other i7 6 core mobile chips.
It may run a bit hotter if you run it at sustained load, but mostly you're paying extra for the higher-efficiency chip that can handle the extra clock speed.
An i7 from the 15W TDP series can still perform slower and use less power than an i5 from the 45W TDP series.
The notebook i9 is still an i9 relative to their notebook processors. It's still a 45W TDP processor, just like the other i7 6 core mobile chips.
It may run a bit hotter if you run it at sustained load, but mostly you're paying extra for the higher-efficiency chip that can handle the extra clock speed.
An i7 from the 15W TDP series can still perform slower and use less power than an i5 from the 45W TDP series.