I didn't see a huge power increase, but I did see some extra top end with the 7th cell. When you add a cell just remember that it goes in series with the others. + to - I don't know what kind of packs you are using. If they are side by sides, just copy what is already there for the 7th cell.
I use a fan on my controller, but it is larger than the heatsink so I use a bracket to hold it above the controller. I am not sure what size it is that would fit like they are saying but you can check places like Jameco Electronics or Digikey
You will have to do some measuring to find out what size you need to fit the way they are saying to do it.
Also, if you are using a JR receiver you will have to plug the fan into the 3rd channel slot since I don't believe the battery slot will supply power, only receive it.