Unity 5 will automatically rewrite references to .particleSystem, .collider, etc to the GetComponent<T> version when you upgrade your project. That won't help when you need to update to a new version of NGUI though (if NGUI is still using .collider). I would say Aren should run his NGUI project through a Unity 5 upgrade now (since GetComponent<Collider>() is no worse than .collider) but leave the caching of transforms until Unity 5.1 or a time when the majority of users are on Unity 5.