We tested for DS at 3.5 only because we had concerns (preschool not going too great, and we wanted to rule out GT, although it ruled it in and as the reason for his issues) - and we had options to turn to when preschool did not work out (he misses cutoff for K by 1 day, and our local public school does not permit early entry). But for his younger sister, we have not tested yet - we have not had to since she is attending the same school as her brother... and because in daycare, her teacher knew exactly what she was dealing with and was able to work with DD at her level so DD did not have same issues that DS had.
If there is no actionable items for you to take if indeed testing shows results you expect, then I am not sure it is worth the time and money.
The center he is in now - would the teacher know what she is dealing with and work with him for the next 3 years (for instance, not repeating same materials/topics each year like some schools do)? It sounded like they already have others like him. You say no gifted schools around, but are there other schools around to look at that would be flexible about age/maturity (for instance some private schools are flexible with cutoff dates, and then after a few years, the kids could to public school at the grade level they were in the private school)?
Just because a school is not specifically for GT does not mean they may not be able to support your child during these years - we had DD in a Montessori school for a year until she was old enough to go to DS' school, and that was a very good fit for her due to the multi-age rooms and activities/pacing.
