Properly tuned, the Howa 1:9 with 69 grain match bullets should be fine at 600 if the wind isn't gusting too much. If you get bad wind gusting, you'll be working harder to compensate for wind deflection than the fellows with faster twists and longer bullets do. I've shot a fair amount of 600 yard service rifle targets with the AR using 80 grain SMK's, but have to agree with the comment that it's marginal when compared to 6.5's, whose long, sleek bullets aren't moves as far by the wind. But then the mouse gun is easier to clean targets with in the rapid fire stages, and because the AR design has better intrinsic accuracy than some other guns used in service rifle matches, the trade-off seems to work out for score.
So, you can see, it's really an issue of what it takes to be competitive in the type of match you are shooting in. For that reason, I would take the previous advice to go be a spectator at a match you are interested in competing in. See what kinds of groups fellows are putting up at the different ranges and what equipment they are using to do it with. For both ammunition cost and ease of practice with large numbers of rounds, the smallest cartridge that is satisfactory is a good choice.
So, you can see, it's really an issue of what it takes to be competitive in the type of match you are shooting in. For that reason, I would take the previous advice to go be a spectator at a match you are interested in competing in. See what kinds of groups fellows are putting up at the different ranges and what equipment they are using to do it with. For both ammunition cost and ease of practice with large numbers of rounds, the smallest cartridge that is satisfactory is a good choice.