See, my group has always figured that heighten spell has a variable level increase... i.e. if you choose 2 levels, it increases by 2, etc.
Therefore, we've been not sure how to handle heighten spell either. Can you take a fireball and do the following:
fireball 3rd level spell.
heightened (2 levels) fireball 4th level spell.
heightened (2 levels) heightened (2 levels) fireball 5th level spell.
heightened (2 levels) heightened (2 levels) heightened (2 levels) fireball 6th level spell.
etc.
This, of course, will all go away with the metamagic stacking changing and already goes away depending on how you handle heighten spell and improved metamagic.