20% down on a $650k house is $130,000. At that point $15,000 one way or the other is a rounding error.
I tend to agree with @KungfuRabbit, rather than taking a loan from your 457, plan to meet any unexpected expenses out of your $2,000 a month of surplus income, with the option of temporarily stopping 457 contributions and roth contributions (so $1,500 * 2 + ~460*2 = $3,920 of additional possible surplus income) if you hit a serious cash crunch.
$15,000 is only 2.5 months of surplus income between your post and pre-tax savings.