I think reading MTH study guides, most of which appear on the scholastic wizard as grade 3 or 4, is a pretty good indicator of some level of giftedness. Along with everything else you mentioned, plus the fact that others in the family are gifted.
Are there other really great K-1 options that you'd rather consider? Is it really that bad of an option? Are there concerns you have that you might be able to fix through advocacy? Are spots in the magnet really tough to get into if you don't go to their K-1 but your DS qualifies?
It's always hard to make decisions about schooling. In my experience, programs can change in a couple of years, so there's not even any guarantee what will be available down the road. You try to make the best decision based on the fact you have. That is why testing information is rather nice to have, as it adds more information to help you make a more informed decision.
