And now back to the body!
A logic board relocation isn't the only easily-recognized layout difference between the iPad Pro and previous iPads. While the iPad Air 2's interior real estate was dominated by a proportionately massive battery, the iPad Pro dedicates a significant amount of room to speaker enclosures.
That's space that could have potentially been used for an extra 50% battery capacity. We're sure Apple was very careful setting the balance between battery capacity, weight, and sound quality.