Should not cause a heavy slow down , i have read of some people
having moving obsticles that dynamicly alters the navmesh
but this all just happens from a bounding box which selects
tiles needed to be rebuild , and simply there cannot be that many
tiles and detail in such a small area of a game made to
perform well ..but we will have to see.

Yes this is more flexible and also easier i want to leave lower
functions for meshes aswell for when needed ..