It doesn't mean they will have a better winning rate. If Klay's comeback, Steve Kerr needs to modify the rotation of the players. That will affect the productivity rate of those players from the bench that also show some improvements while the Warriors do have a good record of 15-2.
On the other hand, if that changes will be made, Klay just needs to fill the hole on those bench players which not be a problem as he is explosive and consistent. But I'm not expecting that he will go back right away at his 100% performance as he will work on catching up on his early games.
Nobody is expecting for him to be 100% on his comeback and nothing is . After all, it happens to all players who incur injuries in the past. Injuries like Torn ACL will take time and even though you are ready to comeback, there is still a chance that it will happen again (hope it will not happen to Klay). With regards to rotation, I believe that Coach Kerr is ready for it. I know that he already created a new rotation when Klay comes back.