I guess I am curious why you would ever need a steerer longer then 300mm? Even with a head tube length of 230mm you can build with a stack height of 35mm with a typical stem clamp length of 35mm.
Haven't seen too many head tubes as long as 230mm.
If you are thinking about a lot of spacer stacking under the stem, keep in mind with a carbon steerer in particular, 40mm is about maximum for structural integrity.
You must be a giant.
George