I think that in general, having an idea of LOG can be important in planning for school. A child might not have yet an academic kick and be very smart. Knowing his/her potential, would prepare you to expect it.
While many gifted kids do well in K, as PP said, 1st grade can be a disaster, when the schools have to focus more in academics and differentiating becomes harder for many teachers.
I would consider testing after he turns 6, to be able to use WISC, or do SB5. Testing at a relative young age (but not too young, preschool age) has advantages for the very gifted kids. Otherwise they can more easily hit ceilings on subtests, and the picture becomes murkier.
