Things change at 500 yards. Perhaps change is not the right word, but little things that can be discounted at 300 yards will take you right off the target at 800. Some of these gremlins include, but are not limited to: mirage, a quartering wind, near wind vs far wind considerations, optical clarity, the scope's resolution, contrast and parallax, the precision and repeatability of the scope's tracking and adjustments, spin drift, the bullet's mach number and stability upon reaching the target, and the uniformity of the ammunition as it pertains to concentricity, neck tension, and variations in velocity.
Each rifle is an individual. It takes time to discover which combination of primer, powder, and bullet will perform the best. When choosing the components for long range ammo, stick with match bullets with secant ogives rather than game bullets. Short cartridges as a rule are less compatible with long bullets when you are shooting a factory rifle with a standard length lead. A custom barrel by contrast can have a lead cut for a specific bullet length. As a result the long bullet must be seated deeply into the powder capacity of the cartridge thus you might not be able obtain the optimal velocity for that bullet weight with a slow burning powder, which is the accepted combination. Therefore you may find that 175-190 gr bullets produce the best balance between powder capacity and velocity. Having said that, when shooting at a known distance, velocity and drop are lessor concerns provided you scope has sufficient adjustment to compensate for the trajectory. Heavy bullets require smaller powder charges, and a smaller powder charge extends the life of the barrel.
Don't worry so much about the 1:10 twist. While is is true that any given twist has an optimal bullet length, any rate of twist will stabilize a wide range of bullet lengths. While I wouldn't expect a 155 gr bullet to produce optimal results, it could, but certainly any match bullet of 175 grs or heavier can be expected to perform well from a quality 1:10 twist barrel. I have never witnessed a quality light bullet becoming unstable due to a fast twist alone, but I have often seen long or heavy for caliber bullets rendered useless due to insufficient spin resulting in large groups and elongated bullet holes.