I've got a 30" barreled 6.5-08... and currently I'm looking down the road about 500-1000 rds to when I'm gonna have to get a new barrel screwed on (around 2500-3000 total rds thru the tube). At the moment, I'm leaning towards either a 26 or 28" tube, probably the 26" w/ a heavier profile. And that's on a gun that gets used out to 600yds regularly, and once in a while further. The previous .260 I had was a Rem Model 7, and I didn't have a chrono back then, so I don't have a good comparison w/ *that* short of a barrel (20" or so). I do however have a pile of data scraped off the internet and most people seem to be getting velocities very similar to what I'm getting w/ a 30" tube... w/ a 26-28" tube. If my gun was Prone-only, or perhaps Benchrest, I might consider running hotter loads w/ slower powder, but as it is, I'm topping out (as far as safety and reasonable case life) at around 2700-2800fps. I can and have pushed it harder, but when I suddenly start getting speeds way above and beyond what most people w/ 30" tubes get... I tend to become a little cautious. Try as I might, I couldn't break 3200fps w/ the 123gr Scenars. 3050-3100 was more the norm w/ sane loades, about what people w/ 26" barrels have been getting. So in retrospect, it *appears* that 26" is probably where the point of diminishing returns begins for this caliber. Not that a person can't run hot loads of slow powders in a longer barrel and get some more speed, but I think the 'improvement' might start becoming minimal... as in 1 or 2 clicks of a 1/4" sight or scope at best, even at 500-600yds. For a hunting 'carry' gun, I'm guessing 22-24" for something most likely to be used out to 300-400yds probably would be a pretty good setup.
HTH,
Monte