I'm sorry, that's so hard when your partner's not on board. Could be that talking it through is all that's needed, if the educational decisions have been up to you so far. Maybe you can find some good articles here to share with her about the benefits of radical acceleration in PG kids and the troubles with not accelerating:

http://www.davidsongifted.org/db/browse_articles_164.aspx

This one is pretty good for explaining the differences between MG and HG: http://www.davidsongifted.org/db/Articles_id_10065.aspx Here's an excerpt:
The social alienation of extraordinarily gifted children is exacerbated by the insistence of educators and parents that they spend most of their time in the company of chronological peers. The assumption that children of the same age constitute a true peer group only holds true for children of average development. The term peer does not, in essence, mean people of the same age, but rather refers to individuals who can interact at an equal level around issues of common interest (Lewis, Young, Brooks, & Michelson, 1975). Highly gifted children are not likely to find developmentally defined peers among their age-mates, and in fact many of them prefer older companions (Hollingworth, 1942, Silverman, in preparation). Given a choice, highly gifted children tend to form friendships with others of similar mental age (O'Shea, 1960).