Personally, I wouldn't count on just supplementing outside of school. If your child continues to test in the EG range, then even the first couple of years of sitting in a class listening to teachers repeat everything ten times could lead to a seriously jaded and/or under-achieving outcome.
Option B sounds much more appealing, the differentiated instruction may be more geared towards the intellectually gifted over the high achieving. You EG child may only need to hear something once to learn it; an average high achieving student may still need simpler explanations with high repetitions. Also, in option B you may be welcomed as an advocating parent trying to look out for their gifted child rather than just another parent trying to push the school for their high achiever.
But everyone has their own read on things and their children, and I've located in a very large district that "gets" gifted. and my DS6 is in a magnet school more resembling your option B.
