I don't use a heating core or Bake Even strips or an upside down flower nail either and don't have problems. But some people like to use them and feel that it makes the centre cook at a faster pace and that you get a more level cake.
People use a greased and floured upside down flower nail that you make roses on, or even a few, in the cake centre to attact the heat to the centre.
I find that you need two cake mixes and a bit more, like a cup or so, to fill this size pan.
Hugs Squirrelly Cakes