what effect does this have on the ball reaction. picked up a used Siege and it has the MB drilled out. the ball seems to roll pretty good with quite a bit of backend. just curious.

It depends on how the mass bias is created...if it is created by an asymmetrical core shape, i.e. length vs width, then drilling out the mass bias increases differential along with lowering RG. If a piece, mass or nugget is used, then it will lower or decrease dynamics, making it more symmetrical.