Agree with HowlerKarma here. One of the hallmarks with my 2e/pg ds8 is that he can make quantum leaps and rapidly accelerate through stuff when motivated and engaged and vested in the material.
Ds did take the g/t test at age 4 when we were living in NYC. He didn't qualify for the program. I don't regret him taking that test though because it was free and it did give us some valuable information.
Within a year, we were living in MA and were being told by a private gifted school that ds was pg and the school could no longer accommodate. He had went through the pre-k/k/1st grade curriculum within 2.5 months. I had withdrawn him from a public, special needs pre-k program for the private gifted school because 1) ds was bouncing off the walls, 2) had started to write three- and four-letter words, and 3) naively thought the private gifted school could accommodate him. I didn't really know that he would go through the curriculum so fast since he hadn't been placed in a more academic setting until that point.
With pg kids, I think you'll find that some kids are ok being in public/private schools. Other kids, like my son, do not tolerate it and need a more individualized education which un/homeschooling affords. I don't know if any testing will help you pinpoint those details. I think it more depends on temperament, internal/external motivation, social/emotional needs, etc.
