I think Peetza has it about right. I get to roughly the same number by a different calculation, and the fact the two agree is a fair sign we are on to something.
What happens with a standard construction bullet is core stripping, where the jacket experiences so much angular acceleration (rotational acceleration) that inertia in the lead core drags hard enough to cause it to slip against the inside of the jacket so it doesn't get up to the full jacket rotation speed. As soon as the bullet clears the muzzle, friction slows the lighter jacket's rotation and, to a lesser degree, speeds up the core, with the two coming into agreement somewhere inbetween the different spin rates they had at the muzzle. Even if the equilibrated spin rate is fast enough to keep the bullet stable, with the engraving of the rifling into the jacket acting like speed bumps on the inside, you can pretty well guess the core shape is affected and that's not good for mass symmetry of the bullet. Any asymmetric bullet mass wobbles badly in flight due to the spin. This opens groups up significantly.
Harold Vaughn used a magnetometer to measure the slip in some 270 bullets, and it turned out to begin at about 3150 fps MV with a 10" twist, and that would be about like an 8" twist in a .224. A 9" twist .224" bullet at 3540 fps would be getting about the same angular acceleration forces at the perimeter, so that's my number that comes out so close to Peetza's.
If you have bonded core bullets you can obviously go faster without causing slip, but I don't know exactly how much. Solids shouldn't suffer the problem at all, but there often is another precision problem. Even if you avoid core slipping, if a bullet isn't made to perfection in mass distribution (and none are, though match bullets typically do best) and slight mass distribution error causes wobble from spinning. The faster the spin or the worse the mass asymmetry, the more wobble. So excess spin can cause the bullet to fly a helical path around the trajectory. Again, not helpful to group size.
As to BC's, they depend on shape aerodynamics and sectional density. You can make a light bullet with low BC by having a lot of hollow space in a long nose form or by using a less dense material than lead, like copper. The current Sierra 155 grain Palma bullet, for example, has a BC about the same as the 175 grain SMK. It has less bearing surface, so it aligns in the bore less well. The result is it often has more yaw at muzzle exit that takes longer to go sleep on the way down range, but the effects of wind and air resistance are the same as for the 175 fired at the same velocity. It just will carry proportionately less energy to the target, which matters to sniper ammunition. Its short bearing surface makes it pickier about alignment in the case neck and chamber, though the Palma guys are equipped to handle that.
