First of all do you really…
First of all do you really think Tesla wouldn't have put the extra battery there if it would have fit?
Second of all it's beyond obvious that the space under the bed is a small fraction of the size of the extra pack they came up with, they need all the volume they can get to provide anything remotely close to the 500 miles of range they initially promised. Even with the huge additional pack taking up over 1/3 of the bed they couldn't hit 500 miles of range. The space under the bed would have barely been good enough for an extra 50 miles of range.