Not necessarily, I worked with a 300Win mag that was an absolute tack driver with factory ammo (we tried 4 different types and 2 shot real well and 2 were average).
I wont argue with that... but... that is probably almost as rare as drawing a sheep tag in Montana.
With all the different 308 ammo available, I wouldn't be surprised if sub 1moa results could be found off the shelf if you had a good rifle. I've found that good rifles shoot just about everything pretty well.
I would think the same could be said for availble 270 WSM ammo, or very close... and yeah, a good shooting rifle is a requisite which I have heard and read nothing but good reports with Tikka 270 WSM's.
I agree that for 800-900yd results, handloading is a more gauranteed approach but you still need a good rifle. I also agree, there are easier ways to shoot that far than with a 308, but with good barrel life and off the shelf ammo, the 308 is very tough to beat.
AJ
Hard to argue with the barrel life of a 308... but I just like the zippier cartridges