First, +1 for Varget - excellent powder for .223 with most bullets.
In my experience, 55 grain bullets, especially Sierra, have shot really well. The issue with 1/9 twist barrels and heavy bullets really has more to do with the length of the bullet rather than the weight - if you plan to shoot heavy bullets, pay close attention to the length/shape of the bullet. The availability and selection of bullets in this caliber is nearly staggering. Have fun - find one that works, then try another, and another, and......