I have a 18" ARP barrel(1:11.25" 5R) that chronographs 95 gr. Barnes TTSX loads at 2850 FPS, with no signs of pressure.
Real world speeds will have more to do with the barrel twist rate, and of course quality, than length of barrel. The early standard barrel twist rate for the 6.8 SPC was 1:10, which was an artifact from the 270 Win. chambered rifles. So enterprising 6.8 fans started playing with barrel twist rates, and found they could use slower rates, like 1:11-1:12, to get better velocities, before signs of pressure showed up, using fairly fast powders. So "real world velocities", in any barrel, will only be a fair comparison, if one adds twist rate of the barrel, to the comparison.
As an example, I have a 6.8 SPC II RRA upper(16" barrel 1:10" twist), that when tested with the same 95 gr. Barnes TTSX loads, showed signs of pressure, so I didn't even chonograph them, as I will have to adjust the load, for this rifle. It is relegated to Home Defense/yard varmints duty, so I have 110 gr. VMax load that is developed for this rifle, and IIRC it chronies around 2600 FPS.
Squeeze