The new version of PhysX is more capable of multithreading across processor cores, meaning that 64 Bit clients for Windows and Mac OSX are more than just a thought in the development process.
Unity 5 will offer a solid performance boost across all operating systems as well as increased performance and stability on 64 Bit platforms. Although the scope of the update will be reduced by a small margin, this is in favor of speeding up the release schedule for the game.
With the recent update to the Unity 5 Engine, Kerbal Space Program developers have moved the release date for the KSP 1.1 update forward.