The 6mm are good out to 600, after that you start giving up too much to the 6.5mm and the 7mm rifles. It's like everything, they are good but they can only take you so far.
You see alot of 6mm at the Midrange championship, 6BRX, Dasher, 6XC, and 6X47 Lapuas to name a few, but at the Longrange Championship there are mainly 6.5mm and 7mm. 6.5x47 Lapua, 6.5-284, 284, 284 Shehane, 7 SAUM, and one or two 7WSM's.
So as far as 6mm being more accurate I would say only out to 600 yds., after that it's necessary to burn more powder!
I have to disagree. There have been many 1k matches won with 6mm's both for score and group. Yes, it gives up a bit to the bigger stuff when the wind really starts howling but the 6mm shouldn't be underestimated.
Some of my best 1k groups have been shot with the puny little 6br improved and many a rockchuck and prairie dog have been killed in excess of 1000 yards with it too.
Out here in the west, the 6mm's show a bit better than they do back east but that hasn't stopped the Williamsport crowd from using and winning with them.