I have the same rifle in left hand and synthetic stock and it is definately capable of hitting deer at 700 yards. I haven't done it yet

, but I'm sure killing the heck out of the paper at 600 lol!
Mine gets most groups in the .5 MOA and ALL groups in the .75 MOA with my best load worked up (5 rds). IMO, you won't see a much more accurate factory rifle out there than the Tikka and Sako. My next long range rifle will be a simi custom job. I use 180 Nosler AB's and H4350 powder pushing to 2915 fps muzzle.
I believe the biggest difference in the barrel thickness is reduction of barrel "wip" while fireing, added weight for steady shooting, and ability to fire more rounds without overheating the barrel. A really thin, really long barrel isn't a good idea, but you aren't going to be shooting more than a couple rounds a time anyway, and the sako barrels are very very good. I borescoped mine when new and it looked as smooth as a Shillen match barrel. However, take what I say w/ a grain of salt because I hunt in MN and most of my shots are under 200 yards, but I could have the occasional shot at up to 800 and I am just trying to get myself to the point of being able to take it

. Point being, many of the guys on the site do much much much more long range shooting than me.