Optimum accuracy demands a perfect base and perfect muzzle crown. Maybe the results might be different if someone could produce a perfect gas check bevel base bullet.
I believe that the above is true because the crown of the muzzle has to "let go" of the base of the bullet in a consistent and repeatable way.
This may be the reason why bevel base can be potentially more accurate than plain base in a cast bullet.
It was once explained to me that in a bottom pour mould, the base of the bullet in contact with the sprue plate often has imperfections due to shrinkage, incomplete fill-out, and temperature variations. If the edges are not sharp and crisp, they will not "let go" of the crown of the muzzle in a uniform fashion.
The advantage of the bevel (even a very small one) is that the edge that is seen by the barrel is the point where the bevel begins its inward inflection, rather than the actual base that cools touching the sprue plate. Minor imperfections in the base will have less of an effect on accuracy because it is not in actual contact with the barrel.
I am neither an engineer nor a metallurgist, but this explanation makes a lot of sense to me.
The disadvantage is one of cost. You need a nose first sizer like the more expensive Star because the "up and down" sizers like the RCBS and Lyman will fill the bevel with lube.
Tom