"Balance" of the gun is never "ruined" by sleeving. Changed, Yes. Ruined, no. Sleeving is likely to give the gun thicker barrel walls than when new, but not necessarily. If you pay to have the sleeves bored and struck to "OE" thickness, then the gun will handle as when new.
Sleeved guns generally gain weight, have more forward balance, and higher swing efforts. For game guns in the hands of modern shooters, these four characteristics are moved in the direction of better shooting.
Keep in mind that there is no such thing as "magic balance" that makes a gun good for all shooters. Like stock dimensions, differing shooters are better serves with differing handling characteristics. Sleeving gives you a chance to make a major change in a gun's handling while restoring safe shooting.
DDA
