Why will Stubbs' SLG go up? He's hit 21 HR in 925 professional ABs and at no point has he been young for his league. He's basically send up flares that power isn't ever going to be a featured part of his game. IMO, projecting a guy to hit better in the majors than he has as a 22- and 23-year-old in A ball is little more than wishful thinking. Sure, it could happen, but I'd think right now if you ignored his name and what organization he played for and just looked at what he's doing, the more responsible projection would be that he's progressing more like he'll .220-.230 hitter in the majors (better breaking pitches are going to drop his BA). Think Ruben Rivera with a little less power and a little more speed. And I'd say that's a fairly rosy outlook. He could just as easily get chewed up by better pitching in the upper minors and never get much more than a cup of coffee in the bigs.
The scenario you've presented is what would happen if Stubbs improved dramatically. The problem is he's no longer at an age where that kind of geometric progression is likely.