Yes, it will. It checks once every second to detect changes in the variable and updates them accordingly.
There's a cached copy of the value because Support_Preferences is designed to operate objectively while RTB preferences were primarily global variables. Directly registering a preference instead of using RTB adds a bunch of callbacks in flexibility to how the pref operates, however I don't think a single add-on uses default preference registration.
A default registered pref has the following callbacks available:
Preference::onUpdate
Preference::onLoad
Preference::onDefault
Where a custom namespace can be set similar to how TCPClient works. Since these are objective, it makes sense to keep the value as such, Preference.value. Support for global variables is integrated throughout, but it's not the primary design.