Iteration planning meetings are seriously expensive.
  Group discussion around design, group estimation, group 
  acceptance, all highly inefficient. [...] I remember just 
  getting bored to tears listening to discussions around 
  stories I wasn’t developing on to begin with.

That's quite team size dependent. In a scrum team with 4 people this isn't a problem - but when the team's twice as large it doubles both the amount of work to plan and the hourly cost to assemble the entire team, quadrupling planning costs.

If a team is structured in such a way that a developer knows they won't be working on a story, that seems like a logical line for splitting into two scrum teams.