The stability problem with the 168-SMK in the transonic range is a dynamic instability that is thought to be due mostly to its 13° boattail angle and the common rates of twist used in .308 bullets. Long-range bullets typically use 8° or 9° boattails. I know someone who has got the 168-SMK's to go to 1000 yards, but was using a cut-down Palma barrel with either a 13" or a 13.5" twist and had zero wind conditions. Faster spin and strong winds seem to be part of the problem the 13° boattail has with transonic velocities.
The Hornady 168-grain open tip match bullet is Hornady's competition for the Sierra design and has the same 13° boattail. However, it has a longer bearing surface and uses a larger ogive radius according to Bryan Litz's measurements, so whether or not it will have exactly the same stability issue at the same velocities or not, I can't say to a certainty. But I would not count on it doing well without testing it. If you can get an 800-yard target and try them out when you have a cross-wind, that will tell the tale. All my attempts with the Sierra in those conditions were turning in keyholes and wouldn't stay on the paper of the standard NRA long range target.