DS turned 5 in June and started K in late August. He was already reading independently and could do all the counting etc they were expected to learn.
His K teacher was awesome! She did her best to differentiate and actually worked with him on his reading (his strongest subject by far). She seemed as excited as we were to see how far he could progress. In some of the metrics like counting to 20 she would just let him go as far as he could, just to see what he could do.
His first grade teacher was awful, and we just had to hold our noses and deal with it. Second grade was pretty good, and I am cautiously optimistic third grade will be more like K.
So really, YMMV, it depends on the teacher and how GT friendly your school system is.
Hope this helps!
