Just finished a .223 for 600 yard and in using a 1-8" twist 28" medium Palma taper, have a .308 in the works for 600 yards and out, with a 1-12" twist 28" medium Palma taper.
IMO (based on personnal experience, others milage or velocity may vary) the extra 2" gains around 50-60 fps so if I hit my target velocity with a less than 30" barrel with a load that isn't running on the ragged edge I'm happy.
Open is another story for me. I very much like my 6.5 X 284 and it pushes a 139 gr Scenar to 2945 fps with no pressure signs out of a 26" barrel in heavy varmint taper with a very high degree of accuracy.
Given the much smaller case of the 6.5 X 47, you'll need as much barrel length as possible to hit 2950 or so with the 140 gr range of bullets if it can be done at all without going overpressure on the load. And the 2950 fps node seems to be the sweet spot for the 140 gr range of bulets. Couldn't tell you much about the lighter bullets in the 120 gr range, although what little experimentation I did with the 123 gr Scenars proved them to be extremely accurate with the 6.5 X 284 at shorter ranges.